<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019cb57f-ceb2-7b11-9822-9b5921a971ea,g:94f2386e-53b2-4ed6-9b15-c20c4074b292-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://fivestarbank.com/role/CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYUnaudited" xlink:type="simple" xlink:href="fsbc-20260331.xsd#CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYUnaudited"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYUnaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019cb57f-d1d9-7710-aa57-f972ec957dce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019cb57f-d1d9-7b81-a6ef-2043caf0dd4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019cb57f-d1d9-7710-aa57-f972ec957dce" xlink:to="loc_us-gaap_StatementTable_019cb57f-d1d9-7b81-a6ef-2043caf0dd4e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019cb57f-d1d9-7b2a-9d79-35cca754b1d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019cb57f-d1d9-7b81-a6ef-2043caf0dd4e" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019cb57f-d1d9-7b2a-9d79-35cca754b1d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019cb57f-d1d9-7b2a-9d79-35cca754b1d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019cb57f-d1d9-7b2a-9d79-35cca754b1d4" xlink:to="loc_us-gaap_EquityComponentDomain_019cb57f-d1d9-7b2a-9d79-35cca754b1d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019cb57f-d1d9-7de0-b7fe-8d530d3f1ef0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019cb57f-d1d9-7b2a-9d79-35cca754b1d4" xlink:to="loc_us-gaap_EquityComponentDomain_019cb57f-d1d9-7de0-b7fe-8d530d3f1ef0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019cb57f-d1d9-7d6e-9406-cbc9d991f42a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019cb57f-d1d9-7de0-b7fe-8d530d3f1ef0" xlink:to="loc_us-gaap_CommonStockMember_019cb57f-d1d9-7d6e-9406-cbc9d991f42a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019cb57f-d1d9-7ccb-81b0-20b4f0bc4992" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019cb57f-d1d9-7de0-b7fe-8d530d3f1ef0" xlink:to="loc_us-gaap_RetainedEarningsMember_019cb57f-d1d9-7ccb-81b0-20b4f0bc4992" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019cb57f-d1d9-71fb-92e5-84933f242bb9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019cb57f-d1d9-7de0-b7fe-8d530d3f1ef0" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019cb57f-d1d9-71fb-92e5-84933f242bb9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cb57f-d1d9-7710-aa57-f972ec957dce" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019cb57f-d1d9-74cb-a9e2-4a3c5dd83ce5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019cb57f-d1d9-74cb-a9e2-4a3c5dd83ce5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019cb57f-d1d9-7aec-aeb6-af1ba15ef155" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:to="loc_us-gaap_StockholdersEquity_019cb57f-d1d9-7aec-aeb6-af1ba15ef155" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019cb57f-d1d9-7495-b1ff-caa955ea6e29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:to="loc_us-gaap_NetIncomeLoss_019cb57f-d1d9-7495-b1ff-caa955ea6e29" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019cb57f-d1d9-7bc5-855c-78613442a823" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019cb57f-d1d9-7bc5-855c-78613442a823" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019cb57f-d1d9-7a16-8e6b-43d4c2995fe4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019cb57f-d1d9-7a16-8e6b-43d4c2995fe4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation_019cb57f-d1d9-7b14-b3b1-5ab4146e07cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:to="loc_us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation_019cb57f-d1d9-7b14-b3b1-5ab4146e07cf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019cb57f-d1d9-74f4-97f3-fadb7d349a6f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019cb57f-d1d9-74f4-97f3-fadb7d349a6f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_019cb57f-d1d9-7e30-a668-23badc6bfb9b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cb57f-d1d9-7aea-9ec3-d19f759984d5" xlink:to="loc_us-gaap_DividendsCommonStock_019cb57f-d1d9-7e30-a668-23badc6bfb9b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019cb57f-d1d9-74f2-8fd7-8e52b14f6669" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019cb57f-d1d9-7d0e-ac65-f9da0f432a84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/CONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited" xlink:type="simple" xlink:href="fsbc-20260331.xsd#CONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019dcc9f-4e4d-7ab9-b667-5a6a60a8bda4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019dcc9f-4e4d-7973-9900-fae369b4e5fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019dcc9f-4e4d-7ab9-b667-5a6a60a8bda4" xlink:to="loc_us-gaap_StatementTable_019dcc9f-4e4d-7973-9900-fae369b4e5fd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_019dcc9f-4e4d-7f25-af77-214d889db869" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019dcc9f-4e4d-7973-9900-fae369b4e5fd" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_019dcc9f-4e4d-7f25-af77-214d889db869" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019dcc9f-4e4d-7f25-af77-214d889db869_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019dcc9f-4e4d-7f25-af77-214d889db869" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019dcc9f-4e4d-7f25-af77-214d889db869_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019dcc9f-4e4d-7c7f-9a43-91a54f6c16f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019dcc9f-4e4d-7f25-af77-214d889db869" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019dcc9f-4e4d-7c7f-9a43-91a54f6c16f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_CA_019dcc9f-4e4d-77fd-aa0e-b25da21ff766" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019dcc9f-4e4d-7c7f-9a43-91a54f6c16f9" xlink:to="loc_stpr_CA_019dcc9f-4e4d-77fd-aa0e-b25da21ff766" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalTaxJurisdictionOtherMember_019dcc9f-4e4d-7265-8b2e-ee21ae20b5c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalTaxJurisdictionOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019dcc9f-4e4d-7c7f-9a43-91a54f6c16f9" xlink:to="loc_us-gaap_StateAndLocalTaxJurisdictionOtherMember_019dcc9f-4e4d-7265-8b2e-ee21ae20b5c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-781e-bbbd-149715ad0617" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019dcc9f-4e4d-7ab9-b667-5a6a60a8bda4" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-781e-bbbd-149715ad0617" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019cb57f-d1d9-73c9-b9df-af5aadc97aec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-781e-bbbd-149715ad0617" xlink:to="loc_us-gaap_NetIncomeLoss_019cb57f-d1d9-73c9-b9df-af5aadc97aec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-781e-bbbd-149715ad0617" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForLoanLeaseAndOtherLosses_019cb57f-d1d9-79cf-ad2b-a3ef23dd4f9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForLoanLeaseAndOtherLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_ProvisionForLoanLeaseAndOtherLosses_019cb57f-d1d9-79cf-ad2b-a3ef23dd4f9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DepreciationDepletionAndAmortizationNonproductionNetting_019cb57f-d1d9-7389-ab96-3bccb158b987" xlink:href="fsbc-20260331.xsd#fsbc_DepreciationDepletionAndAmortizationNonproductionNetting"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_fsbc_DepreciationDepletionAndAmortizationNonproductionNetting_019cb57f-d1d9-7389-ab96-3bccb158b987" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfDeferredLoanOriginationFeesNet_019cb57f-d1d9-7933-9260-88a12d67a27a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfDeferredLoanOriginationFeesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_AmortizationOfDeferredLoanOriginationFeesNet_019cb57f-d1d9-7933-9260-88a12d67a27a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_019cb57f-d1d9-78f6-b4ed-b6e5691643f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_019cb57f-d1d9-78f6-b4ed-b6e5691643f0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_019cb57f-d1d9-772f-8a3c-5a8862757f6a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_019cb57f-d1d9-772f-8a3c-5a8862757f6a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization_019cb57f-d1d9-71ed-903b-f230ca34048d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization_019cb57f-d1d9-71ed-903b-f230ca34048d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019cb57f-d1d9-742b-a388-cbeacec362f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_ShareBasedCompensation_019cb57f-d1d9-742b-a388-cbeacec362f7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankOwnedLifeInsuranceIncome_019cb57f-d1d9-76e5-9e7e-ea37bd4d87d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankOwnedLifeInsuranceIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_BankOwnedLifeInsuranceIncome_019cb57f-d1d9-76e5-9e7e-ea37bd4d87d9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PaymentsForTransferableTaxCredits_019d8659-99d0-79e4-9d9b-e9e8c727380a" xlink:href="fsbc-20260331.xsd#fsbc_PaymentsForTransferableTaxCredits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_fsbc_PaymentsForTransferableTaxCredits_019d8659-99d0-79e4-9d9b-e9e8c727380a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForOriginationAndPurchasesOfLoansHeldForSale_019cb57f-d1d9-72aa-8926-3715516f3263" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForOriginationAndPurchasesOfLoansHeldForSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_PaymentsForOriginationAndPurchasesOfLoansHeldForSale_019cb57f-d1d9-72aa-8926-3715516f3263" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSalesOfLoansNet_019cb57f-d1d9-76ac-8267-d798504f4fd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSalesOfLoansNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_GainLossOnSalesOfLoansNet_019cb57f-d1d9-76ac-8267-d798504f4fd5" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfLoansHeldForSale_019cb57f-d1d9-71d0-952b-1582082168d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfLoansHeldForSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_ProceedsFromSaleOfLoansHeldForSale_019cb57f-d1d9-71d0-952b-1582082168d4" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions_019cb57f-d1d9-7b2a-bb2a-d6cb4d148e10" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-7d2f-8710-0095ac79ac90" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestmentsNetOfDividendsOrDistributions_019cb57f-d1d9-7b2a-bb2a-d6cb4d148e10" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019cb57f-d1d9-777c-9fc6-805f61d13da1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019cb57f-d1d9-781e-bbbd-149715ad0617" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019cb57f-d1d9-777c-9fc6-805f61d13da1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet_019cb57f-d1d9-785b-93db-575e742207fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019cb57f-d1d9-777c-9fc6-805f61d13da1" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet_019cb57f-d1d9-785b-93db-575e742207fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInInterestPayableNet_019cb57f-d1d9-7df7-9451-355104ef5eaa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInInterestPayableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019cb57f-d1d9-777c-9fc6-805f61d13da1" xlink:to="loc_us-gaap_IncreaseDecreaseInInterestPayableNet_019cb57f-d1d9-7df7-9451-355104ef5eaa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingLeaseLiability_019cb57f-d1d9-7deb-b803-cf9c407e3c8a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019cb57f-d1d9-777c-9fc6-805f61d13da1" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingLeaseLiability_019cb57f-d1d9-7deb-b803-cf9c407e3c8a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019cb57f-d1d9-7d9f-92b6-3a9126b7109c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019cb57f-d1d9-777c-9fc6-805f61d13da1" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019cb57f-d1d9-7d9f-92b6-3a9126b7109c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019dcc9f-4e4d-7ab9-b667-5a6a60a8bda4" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities_019cb57f-d1d9-7c40-9f0a-12f4355ce047" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:to="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities_019cb57f-d1d9-7c40-9f0a-12f4355ce047" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PaymentsForCapitalCallForEquityInvestment_019cb57f-d1d9-70da-8a32-5fc5d589d89f" xlink:href="fsbc-20260331.xsd#fsbc_PaymentsForCapitalCallForEquityInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:to="loc_fsbc_PaymentsForCapitalCallForEquityInvestment_019cb57f-d1d9-70da-8a32-5fc5d589d89f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfEquityMethodInvestments_019cb57f-d1d9-73e1-b371-58cd9f3c3e92" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:to="loc_us-gaap_ProceedsFromSaleOfEquityMethodInvestments_019cb57f-d1d9-73e1-b371-58cd9f3c3e92" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ProceedsFromPaymentsForLowIncomeHousingCredits_019cb57f-d1d9-719d-ad54-bd325a074ea3" xlink:href="fsbc-20260331.xsd#fsbc_ProceedsFromPaymentsForLowIncomeHousingCredits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:to="loc_fsbc_ProceedsFromPaymentsForLowIncomeHousingCredits_019cb57f-d1d9-719d-ad54-bd325a074ea3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromPaymentsForInInterestBearingDepositsInBanks_019cb57f-d1d9-768c-bc7c-53fbf20ae4f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromPaymentsForInInterestBearingDepositsInBanks"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:to="loc_us-gaap_ProceedsFromPaymentsForInInterestBearingDepositsInBanks_019cb57f-d1d9-768c-bc7c-53fbf20ae4f4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PaymentsForOriginationOfLoansNetOfProceedsFromRepayments_019cb57f-d1d9-74b2-9115-9b9823b5986e" xlink:href="fsbc-20260331.xsd#fsbc_PaymentsForOriginationOfLoansNetOfProceedsFromRepayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:to="loc_fsbc_PaymentsForOriginationOfLoansNetOfProceedsFromRepayments_019cb57f-d1d9-74b2-9115-9b9823b5986e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019cb57f-d1d9-7697-9688-392675c8cf7b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019cb57f-d1d9-7697-9688-392675c8cf7b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireLifeInsurancePolicies_019cb57f-d1d9-7cbe-a67c-e818dc9d3fa8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireLifeInsurancePolicies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:to="loc_us-gaap_PaymentsToAcquireLifeInsurancePolicies_019cb57f-d1d9-7cbe-a67c-e818dc9d3fa8" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019cb57f-d1d9-71b8-adff-6b4e829861f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019cb57f-d1d9-7754-9508-6e880d698d46" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019cb57f-d1d9-71b8-adff-6b4e829861f8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019cb57f-d1d9-7dd5-944f-506020594d11" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019dcc9f-4e4d-7ab9-b667-5a6a60a8bda4" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019cb57f-d1d9-7dd5-944f-506020594d11" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeposits_019cb57f-d1d9-781e-bafa-ceac2a7e6b45" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019cb57f-d1d9-7dd5-944f-506020594d11" xlink:to="loc_us-gaap_IncreaseDecreaseInDeposits_019cb57f-d1d9-781e-bafa-ceac2a7e6b45" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDividends_019cb57f-d1d9-761c-852c-7bfa618da921" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019cb57f-d1d9-7dd5-944f-506020594d11" xlink:to="loc_us-gaap_PaymentsOfDividends_019cb57f-d1d9-761c-852c-7bfa618da921" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019cb57f-d1d9-7f7b-8488-b16085895f4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019cb57f-d1d9-7dd5-944f-506020594d11" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019cb57f-d1d9-7f7b-8488-b16085895f4d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019cb57f-d1d9-75f6-9973-cad9307e3ff4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019dcc9f-4e4d-7ab9-b667-5a6a60a8bda4" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019cb57f-d1d9-75f6-9973-cad9307e3ff4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019cb57f-d1d9-79a1-817f-40c6d3947d41" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019dcc9f-4e4d-7ab9-b667-5a6a60a8bda4" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019cb57f-d1d9-79a1-817f-40c6d3947d41" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019cb57f-d1d9-7d11-b9dd-3e03434db5c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_019cb57f-d1d9-7d77-9c10-c69887bb7c58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SupplementalCashFlowInformationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019dcc9f-4e4d-7ab9-b667-5a6a60a8bda4" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_019cb57f-d1d9-7d77-9c10-c69887bb7c58" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_019cb57f-d1d9-70df-aa8b-05ab7b7e205c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019cb57f-d1d9-7d77-9c10-c69887bb7c58" xlink:to="loc_us-gaap_InterestPaidNet_019cb57f-d1d9-70df-aa8b-05ab7b7e205c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNetAbstract_019dcca0-93f4-7b14-b4f7-58a8ae22131b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaidNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019cb57f-d1d9-7d77-9c10-c69887bb7c58" xlink:to="loc_us-gaap_IncomeTaxesPaidNetAbstract_019dcca0-93f4-7b14-b4f7-58a8ae22131b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_019dcc9e-cb53-7e8e-88ef-f9f007993e4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidFederalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxesPaidNetAbstract_019dcca0-93f4-7b14-b4f7-58a8ae22131b" xlink:to="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_019dcc9e-cb53-7e8e-88ef-f9f007993e4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceivedAbstract_019dcca1-1436-750f-8473-22c7f080fd14" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceivedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxesPaidNetAbstract_019dcca0-93f4-7b14-b4f7-58a8ae22131b" xlink:to="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceivedAbstract_019dcca1-1436-750f-8473-22c7f080fd14" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_019dcc9e-cb53-7a5f-8b1e-f36b66bd850c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceivedAbstract_019dcca1-1436-750f-8473-22c7f080fd14" xlink:to="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_019dcc9e-cb53-7a5f-8b1e-f36b66bd850c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_019cb57f-d1d9-7969-a722-c520c3fc2728" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019cb57f-d1d9-7d77-9c10-c69887bb7c58" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_019cb57f-d1d9-7969-a722-c520c3fc2728" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_019cb57f-d1d9-7ca8-8f94-dec8c76ca87e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncashInvestingAndFinancingItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019dcc9f-4e4d-7ab9-b667-5a6a60a8bda4" xlink:to="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_019cb57f-d1d9-7ca8-8f94-dec8c76ca87e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_TransferFromLoansHeldForSaleToLoansHeldForInvestment_019cb57f-d1d9-7da6-aa70-05eaad465cbb" xlink:href="fsbc-20260331.xsd#fsbc_TransferFromLoansHeldForSaleToLoansHeldForInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_019cb57f-d1d9-7ca8-8f94-dec8c76ca87e" xlink:to="loc_fsbc_TransferFromLoansHeldForSaleToLoansHeldForInvestment_019cb57f-d1d9-7da6-aa70-05eaad465cbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrealizedGainLossOnInvestments_019cb57f-d1d9-74e4-ac81-6ed37816adc7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrealizedGainLossOnInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoncashInvestingAndFinancingItemsAbstract_019cb57f-d1d9-7ca8-8f94-dec8c76ca87e" xlink:to="loc_us-gaap_UnrealizedGainLossOnInvestments_019cb57f-d1d9-74e4-ac81-6ed37816adc7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesFairValueofAssetsandLiabilitiesMeasuredonRecurringBasisDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#FairValueofAssetsandLiabilitiesFairValueofAssetsandLiabilitiesMeasuredonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesFairValueofAssetsandLiabilitiesMeasuredonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019cb57f-d1d9-7d6e-8ba5-4741af8a81a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019cb57f-d1d9-7edf-9259-807480e9f2bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019cb57f-d1d9-7d6e-8ba5-4741af8a81a8" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019cb57f-d1d9-7edf-9259-807480e9f2bc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019cb57f-d1d9-713f-8c66-1e4e2ccbdd8a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019cb57f-d1d9-7edf-9259-807480e9f2bc" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019cb57f-d1d9-713f-8c66-1e4e2ccbdd8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019cb57f-d1d9-713f-8c66-1e4e2ccbdd8a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019cb57f-d1d9-713f-8c66-1e4e2ccbdd8a" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019cb57f-d1d9-713f-8c66-1e4e2ccbdd8a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019cb57f-d1d9-7da9-99b9-99e5e5b30992" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019cb57f-d1d9-713f-8c66-1e4e2ccbdd8a" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019cb57f-d1d9-7da9-99b9-99e5e5b30992" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_019cb57f-d1d9-7da9-a018-4f8cecb2e590" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_019cb57f-d1d9-7da9-99b9-99e5e5b30992" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_019cb57f-d1d9-7da9-a018-4f8cecb2e590" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cb57f-d1d9-7c23-9909-b8036b01cd5e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019cb57f-d1d9-7edf-9259-807480e9f2bc" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cb57f-d1d9-7c23-9909-b8036b01cd5e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-7c23-9909-b8036b01cd5e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cb57f-d1d9-7c23-9909-b8036b01cd5e" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-7c23-9909-b8036b01cd5e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-7af6-bc60-774624d0c74b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cb57f-d1d9-7c23-9909-b8036b01cd5e" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-7af6-bc60-774624d0c74b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019cb57f-d1d9-7e2e-8122-bd72e994873a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-7af6-bc60-774624d0c74b" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019cb57f-d1d9-7e2e-8122-bd72e994873a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019cb57f-d1d9-7473-b796-403fcffea274" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-7af6-bc60-774624d0c74b" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019cb57f-d1d9-7473-b796-403fcffea274" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019cb57f-d1d9-7184-af85-a1d6e09f3188" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-7af6-bc60-774624d0c74b" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019cb57f-d1d9-7184-af85-a1d6e09f3188" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-7267-9caa-178a0564560b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019cb57f-d1d9-7d6e-8ba5-4741af8a81a8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-7267-9caa-178a0564560b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesFairValueEstimatesforFinancialInstrumentsbyBalanceSheetGroupingDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#FairValueofAssetsandLiabilitiesFairValueEstimatesforFinancialInstrumentsbyBalanceSheetGroupingDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/FairValueofAssetsandLiabilitiesFairValueEstimatesforFinancialInstrumentsbyBalanceSheetGroupingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019cb57f-d1d9-75ef-8f52-f4a2d5a95637" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019cb57f-d1d9-71f1-868d-6ea9e4ee4510" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019cb57f-d1d9-75ef-8f52-f4a2d5a95637" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019cb57f-d1d9-71f1-868d-6ea9e4ee4510" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019cb57f-d1d9-7ab6-b995-e717c0ed5ab3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019cb57f-d1d9-71f1-868d-6ea9e4ee4510" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019cb57f-d1d9-7ab6-b995-e717c0ed5ab3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019cb57f-d1d9-7ab6-b995-e717c0ed5ab3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019cb57f-d1d9-7ab6-b995-e717c0ed5ab3" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019cb57f-d1d9-7ab6-b995-e717c0ed5ab3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019cb57f-d1d9-7a60-92a7-b2fe8d53d3b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019cb57f-d1d9-7ab6-b995-e717c0ed5ab3" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019cb57f-d1d9-7a60-92a7-b2fe8d53d3b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019cb57f-d1d9-7dd2-b63f-7b91b40034fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019cb57f-d1d9-7a60-92a7-b2fe8d53d3b5" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019cb57f-d1d9-7dd2-b63f-7b91b40034fb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019cb57f-d1d9-7234-a106-293f38a5f1cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019cb57f-d1d9-7dd2-b63f-7b91b40034fb" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019cb57f-d1d9-7234-a106-293f38a5f1cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019cb57f-d1d9-7d73-8d55-949f786f2ceb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019cb57f-d1d9-7dd2-b63f-7b91b40034fb" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019cb57f-d1d9-7d73-8d55-949f786f2ceb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cb57f-d1d9-7d07-97a2-602e97e8db23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019cb57f-d1d9-71f1-868d-6ea9e4ee4510" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cb57f-d1d9-7d07-97a2-602e97e8db23" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-7d07-97a2-602e97e8db23_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cb57f-d1d9-7d07-97a2-602e97e8db23" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-7d07-97a2-602e97e8db23_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-722e-a9e5-ebdf46ce64f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cb57f-d1d9-7d07-97a2-602e97e8db23" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-722e-a9e5-ebdf46ce64f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019cb57f-d1d9-75b4-aed8-3a9f0268fabe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-722e-a9e5-ebdf46ce64f5" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019cb57f-d1d9-75b4-aed8-3a9f0268fabe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019cb57f-d1d9-742e-b11f-726fd862c603" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-722e-a9e5-ebdf46ce64f5" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019cb57f-d1d9-742e-b11f-726fd862c603" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019cb57f-d1d9-759d-9cf0-9a2957fb8bd2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cb57f-d1d9-722e-a9e5-ebdf46ce64f5" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019cb57f-d1d9-759d-9cf0-9a2957fb8bd2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7d7b-8336-be8749d15608" xlink:href="fsbc-20260331.xsd#fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019cb57f-d1d9-75ef-8f52-f4a2d5a95637" xlink:to="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7d7b-8336-be8749d15608" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019cb57f-d1d9-75a9-b82b-840860725e15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7d7b-8336-be8749d15608" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019cb57f-d1d9-75a9-b82b-840860725e15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_TimeDepositAtFairValueDisclosure_019cb57f-d1d9-7fdf-96f0-3f30ab0f59cb" xlink:href="fsbc-20260331.xsd#fsbc_TimeDepositAtFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7d7b-8336-be8749d15608" xlink:to="loc_fsbc_TimeDepositAtFairValueDisclosure_019cb57f-d1d9-7fdf-96f0-3f30ab0f59cb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-76ed-8445-1e718af41133" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7d7b-8336-be8749d15608" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-76ed-8445-1e718af41133" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019cb57f-d1d9-728f-88e5-666cbf893728" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7d7b-8336-be8749d15608" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019cb57f-d1d9-728f-88e5-666cbf893728" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableFairValueDisclosure_019cb57f-d1d9-72b4-8892-96524bf0b6ed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialAssetsRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7d7b-8336-be8749d15608" xlink:to="loc_us-gaap_LoansReceivableFairValueDisclosure_019cb57f-d1d9-72b4-8892-96524bf0b6ed" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7dca-bca5-0a65aa6f0e72" xlink:href="fsbc-20260331.xsd#fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019cb57f-d1d9-75ef-8f52-f4a2d5a95637" xlink:to="loc_fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7dca-bca5-0a65aa6f0e72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimeDeposits_019cb57f-d1d9-7391-88f7-d7c9ca920547" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimeDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7dca-bca5-0a65aa6f0e72" xlink:to="loc_us-gaap_TimeDeposits_019cb57f-d1d9-7391-88f7-d7c9ca920547" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations_019cb57f-d1d9-7808-a7b6-2ddc74184daa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FinancialLiabilitiesRecordedAtAmortizedCostAbstract_019cb57f-d1d9-7dca-bca5-0a65aa6f0e72" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations_019cb57f-d1d9-7808-a7b6-2ddc74184daa" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesAmortizedCostandFairValueofSecuritiesHeldtoMaturityDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#InvestmentSecuritiesAmortizedCostandFairValueofSecuritiesHeldtoMaturityDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesAmortizedCostandFairValueofSecuritiesHeldtoMaturityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-735c-aacb-068034cb3b79" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7135-8d75-09910fdda442" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-735c-aacb-068034cb3b79" xlink:to="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7135-8d75-09910fdda442" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7f2f-9254-1b3276f90484" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7135-8d75-09910fdda442" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7f2f-9254-1b3276f90484" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7f2f-9254-1b3276f90484_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7f2f-9254-1b3276f90484" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7f2f-9254-1b3276f90484_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-738c-bbd0-17ab99fb8b02" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7f2f-9254-1b3276f90484" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-738c-bbd0-17ab99fb8b02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019cb57f-d1d9-7ef6-885e-59e0f39f23d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-738c-bbd0-17ab99fb8b02" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019cb57f-d1d9-7ef6-885e-59e0f39f23d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss_019cb57f-d1d9-7c81-becc-4725fd373d12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-735c-aacb-068034cb3b79" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLoss_019cb57f-d1d9-7c81-becc-4725fd373d12" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecurities_019cb57f-d1d9-7fe2-b538-5d08c7569c8a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-735c-aacb-068034cb3b79" xlink:to="loc_us-gaap_HeldToMaturitySecurities_019cb57f-d1d9-7fe2-b538-5d08c7569c8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain_019cb57f-d1d9-7df3-a5e1-95c6fcc2dd4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-735c-aacb-068034cb3b79" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain_019cb57f-d1d9-7df3-a5e1-95c6fcc2dd4d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss_019cb57f-d1d9-76b5-95e6-36c2f7b0a06e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-735c-aacb-068034cb3b79" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss_019cb57f-d1d9-76b5-95e6-36c2f7b0a06e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019cb57f-d1d9-763e-9bc6-73222acccf3d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-735c-aacb-068034cb3b79" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019cb57f-d1d9-763e-9bc6-73222acccf3d" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesAmortizedCostandFairValueofSecuritiesAvailableforSaleDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#InvestmentSecuritiesAmortizedCostandFairValueofSecuritiesAvailableforSaleDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesAmortizedCostandFairValueofSecuritiesAvailableforSaleDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7d92-9a07-ffeb309b5177" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7f38-84e0-8af2ef75c6d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7d92-9a07-ffeb309b5177" xlink:to="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7f38-84e0-8af2ef75c6d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7cf1-91ed-ef2ed152a5f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7f38-84e0-8af2ef75c6d7" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7cf1-91ed-ef2ed152a5f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7cf1-91ed-ef2ed152a5f3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7cf1-91ed-ef2ed152a5f3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7cf1-91ed-ef2ed152a5f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-76bd-8df4-36bdb7b7256f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7cf1-91ed-ef2ed152a5f3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-76bd-8df4-36bdb7b7256f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019cb57f-d1d9-75d9-8f3e-6abf558c189a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-76bd-8df4-36bdb7b7256f" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019cb57f-d1d9-75d9-8f3e-6abf558c189a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cb57f-d1d9-75b5-9f96-eca4cc8c1f32" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-76bd-8df4-36bdb7b7256f" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cb57f-d1d9-75b5-9f96-eca4cc8c1f32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019cb57f-d1d9-77fa-96db-9b1d799200f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-76bd-8df4-36bdb7b7256f" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019cb57f-d1d9-77fa-96db-9b1d799200f8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralizedDebtObligationsMember_019cb57f-d1d9-77b4-b031-4cfa769c6aad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollateralizedDebtObligationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-76bd-8df4-36bdb7b7256f" xlink:to="loc_us-gaap_CollateralizedDebtObligationsMember_019cb57f-d1d9-77b4-b031-4cfa769c6aad" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019cb57f-d1d9-73aa-9a86-baf5b3bd3c79" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-76bd-8df4-36bdb7b7256f" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019cb57f-d1d9-73aa-9a86-baf5b3bd3c79" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019cb57f-d1d9-7c71-9fdd-c349f1e2e3fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7d92-9a07-ffeb309b5177" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019cb57f-d1d9-7c71-9fdd-c349f1e2e3fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019cb57f-d1d9-7f48-b44f-afa812176416" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7d92-9a07-ffeb309b5177" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019cb57f-d1d9-7f48-b44f-afa812176416" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019cb57f-d1d9-7c4a-b68f-6834ec6ccabf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7d92-9a07-ffeb309b5177" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019cb57f-d1d9-7c4a-b68f-6834ec6ccabf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-7581-b9e2-d36863109f7e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7d92-9a07-ffeb309b5177" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-7581-b9e2-d36863109f7e" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesAmortizedCostandFairValueofInvestmentDebtSecuritiesbyContractualMaturityDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#InvestmentSecuritiesAmortizedCostandFairValueofInvestmentDebtSecuritiesbyContractualMaturityDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesAmortizedCostandFairValueofInvestmentDebtSecuritiesbyContractualMaturityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7ceb-b446-823d437284ba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-768a-b123-4e0ee4732c73" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7ceb-b446-823d437284ba" xlink:to="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-768a-b123-4e0ee4732c73" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7599-a517-133cfa7008ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-768a-b123-4e0ee4732c73" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7599-a517-133cfa7008ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7599-a517-133cfa7008ff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7599-a517-133cfa7008ff" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7599-a517-133cfa7008ff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7a38-9935-19e4415045da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-7599-a517-133cfa7008ff" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7a38-9935-19e4415045da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019cb57f-d1d9-78e6-9cce-2e59d5c45e35" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7a38-9935-19e4415045da" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019cb57f-d1d9-78e6-9cce-2e59d5c45e35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cb57f-d1d9-730a-94c9-7c2c2e011e0e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7a38-9935-19e4415045da" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cb57f-d1d9-730a-94c9-7c2c2e011e0e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralizedDebtObligationsMember_019cb57f-d1d9-76c1-b5b5-ba1a36bd0989" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollateralizedDebtObligationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7a38-9935-19e4415045da" xlink:to="loc_us-gaap_CollateralizedDebtObligationsMember_019cb57f-d1d9-76c1-b5b5-ba1a36bd0989" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019cb57f-d1d9-740a-a594-bcefeefa6d82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7a38-9935-19e4415045da" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019cb57f-d1d9-740a-a594-bcefeefa6d82" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract_019cb57f-d1d9-74fd-8e83-3b0d4868c03c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7ceb-b446-823d437284ba" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract_019cb57f-d1d9-74fd-8e83-3b0d4868c03c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-781a-882c-bf3a90daee41" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract_019cb57f-d1d9-74fd-8e83-3b0d4868c03c" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-781a-882c-bf3a90daee41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount_019cb57f-d1d9-7b91-8883-fdc9f835e8e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-781a-882c-bf3a90daee41" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount_019cb57f-d1d9-7b91-8883-fdc9f835e8e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount_019cb57f-d1d9-7109-8f51-e61049a2ed2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-781a-882c-bf3a90daee41" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount_019cb57f-d1d9-7109-8f51-e61049a2ed2a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount_019cb57f-d1d9-7241-a525-c291544716ed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-781a-882c-bf3a90daee41" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount_019cb57f-d1d9-7241-a525-c291544716ed" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount_019cb57f-d1d9-75c2-8c5e-1788f3c70a93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-781a-882c-bf3a90daee41" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount_019cb57f-d1d9-75c2-8c5e-1788f3c70a93" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_019cb57f-d1d9-7c5f-b34a-d4cc1d3acd72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAbstract_019cb57f-d1d9-74fd-8e83-3b0d4868c03c" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_019cb57f-d1d9-7c5f-b34a-d4cc1d3acd72" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue_019cb57f-d1d9-7442-97e8-3e57a8ce242f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_019cb57f-d1d9-7c5f-b34a-d4cc1d3acd72" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue_019cb57f-d1d9-7442-97e8-3e57a8ce242f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_019cb57f-d1d9-73da-b76c-5f4395d48695" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_019cb57f-d1d9-7c5f-b34a-d4cc1d3acd72" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_019cb57f-d1d9-73da-b76c-5f4395d48695" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_019cb57f-d1d9-7476-b68e-003299fac6e3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_019cb57f-d1d9-7c5f-b34a-d4cc1d3acd72" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_019cb57f-d1d9-7476-b68e-003299fac6e3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue_019cb57f-d1d9-762b-9f28-65dd4c75351a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValueAbstract_019cb57f-d1d9-7c5f-b34a-d4cc1d3acd72" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue_019cb57f-d1d9-762b-9f28-65dd4c75351a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract_019cb57f-d1d9-71c2-97f7-be5d9db1eaf9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7ceb-b446-823d437284ba" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract_019cb57f-d1d9-71c2-97f7-be5d9db1eaf9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-78ea-acaf-4490344cd21b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract_019cb57f-d1d9-71c2-97f7-be5d9db1eaf9" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-78ea-acaf-4490344cd21b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_019cb57f-d1d9-7613-bb32-78587848edf1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-78ea-acaf-4490344cd21b" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_019cb57f-d1d9-7613-bb32-78587848edf1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_019cb57f-d1d9-7f89-85f4-957762c9224a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-78ea-acaf-4490344cd21b" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_019cb57f-d1d9-7f89-85f4-957762c9224a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost_019cb57f-d1d9-7af5-ad61-de026f46f4f6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-78ea-acaf-4490344cd21b" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost_019cb57f-d1d9-7af5-ad61-de026f46f4f6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost_019cb57f-d1d9-7131-9fd2-653be08506dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019cb57f-d1d9-78ea-acaf-4490344cd21b" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost_019cb57f-d1d9-7131-9fd2-653be08506dd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019cb57f-d1d9-7025-a235-5537a42229ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAbstract_019cb57f-d1d9-71c2-97f7-be5d9db1eaf9" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019cb57f-d1d9-7025-a235-5537a42229ab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019cb57f-d1d9-7730-971e-1f18e0dd96cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019cb57f-d1d9-7025-a235-5537a42229ab" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019cb57f-d1d9-7730-971e-1f18e0dd96cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_019cb57f-d1d9-7b5e-87ae-2482524cb8da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019cb57f-d1d9-7025-a235-5537a42229ab" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_019cb57f-d1d9-7b5e-87ae-2482524cb8da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_019cb57f-d1d9-7c4f-996f-466a76bb472e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019cb57f-d1d9-7025-a235-5537a42229ab" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue_019cb57f-d1d9-7c4f-996f-466a76bb472e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue_019cb57f-d1d9-7de6-a96a-13d2135609e3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019cb57f-d1d9-7025-a235-5537a42229ab" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue_019cb57f-d1d9-7de6-a96a-13d2135609e3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecurities_019cb57f-d1d9-7091-94aa-3853a8625dcf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7ceb-b446-823d437284ba" xlink:to="loc_us-gaap_HeldToMaturitySecurities_019cb57f-d1d9-7091-94aa-3853a8625dcf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019cb57f-d1d9-7021-a814-f0048e30e352" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7ceb-b446-823d437284ba" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019cb57f-d1d9-7021-a814-f0048e30e352" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019cb57f-d1d9-7d2f-ace9-3ca615932c0c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7ceb-b446-823d437284ba" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019cb57f-d1d9-7d2f-ace9-3ca615932c0c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-75ab-ae9a-68b30fd50601" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7ceb-b446-823d437284ba" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-75ab-ae9a-68b30fd50601" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesPledgedInvestmentSecuritiesDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#InvestmentSecuritiesPledgedInvestmentSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesPledgedInvestmentSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7fed-aa85-0e1c5f267d1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7390-a756-ec6cf3c734ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7fed-aa85-0e1c5f267d1e" xlink:to="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7390-a756-ec6cf3c734ad" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019cb57f-d1d9-7d5b-9cc9-1b02f4f52d2f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7390-a756-ec6cf3c734ad" xlink:to="loc_srt_StatementGeographicalAxis_019cb57f-d1d9-7d5b-9cc9-1b02f4f52d2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019cb57f-d1d9-7d5b-9cc9-1b02f4f52d2f_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019cb57f-d1d9-7d5b-9cc9-1b02f4f52d2f" xlink:to="loc_srt_SegmentGeographicalDomain_019cb57f-d1d9-7d5b-9cc9-1b02f4f52d2f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019cb57f-d1d9-7ceb-908d-49767c099d41" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019cb57f-d1d9-7d5b-9cc9-1b02f4f52d2f" xlink:to="loc_srt_SegmentGeographicalDomain_019cb57f-d1d9-7ceb-908d-49767c099d41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_CA_019cb57f-d1d9-7bba-974f-a036a7c59ef3" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019cb57f-d1d9-7ceb-908d-49767c099d41" xlink:to="loc_stpr_CA_019cb57f-d1d9-7bba-974f-a036a7c59ef3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PledgingPurposeAxis_019cb57f-d1d9-7252-8e24-84e5e4daa1f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7390-a756-ec6cf3c734ad" xlink:to="loc_us-gaap_PledgingPurposeAxis_019cb57f-d1d9-7252-8e24-84e5e4daa1f8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PledgingPurposeDomain_019cb57f-d1d9-7252-8e24-84e5e4daa1f8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PledgingPurposeAxis_019cb57f-d1d9-7252-8e24-84e5e4daa1f8" xlink:to="loc_us-gaap_PledgingPurposeDomain_019cb57f-d1d9-7252-8e24-84e5e4daa1f8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PledgingPurposeDomain_019cb57f-d1d9-7572-b784-92a9f7816bbf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PledgingPurposeAxis_019cb57f-d1d9-7252-8e24-84e5e4daa1f8" xlink:to="loc_us-gaap_PledgingPurposeDomain_019cb57f-d1d9-7572-b784-92a9f7816bbf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepositsMember_019cb57f-d1d9-7bc5-9631-66551f457ca3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepositsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PledgingPurposeDomain_019cb57f-d1d9-7572-b784-92a9f7816bbf" xlink:to="loc_us-gaap_DepositsMember_019cb57f-d1d9-7bc5-9631-66551f457ca3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalFundsPurchasedMember_019cb57f-d1d9-74eb-9b57-00575d6cdb66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalFundsPurchasedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PledgingPurposeDomain_019cb57f-d1d9-7572-b784-92a9f7816bbf" xlink:to="loc_us-gaap_FederalFundsPurchasedMember_019cb57f-d1d9-74eb-9b57-00575d6cdb66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DepositsAndFederalFundsPurchasedMember_019cb57f-d1d9-712e-90d2-fc6e6d1af38d" xlink:href="fsbc-20260331.xsd#fsbc_DepositsAndFederalFundsPurchasedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PledgingPurposeDomain_019cb57f-d1d9-7572-b784-92a9f7816bbf" xlink:to="loc_fsbc_DepositsAndFederalFundsPurchasedMember_019cb57f-d1d9-712e-90d2-fc6e6d1af38d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-7b6e-a868-f2ee57f8db83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7fed-aa85-0e1c5f267d1e" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019cb57f-d1d9-7b6e-a868-f2ee57f8db83" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InvestmentSecuritiesAvailableforSaleSecuritiesinaContinuousUnrealizedLossPositionDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#InvestmentSecuritiesAvailableforSaleSecuritiesinaContinuousUnrealizedLossPositionDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InvestmentSecuritiesAvailableforSaleSecuritiesinaContinuousUnrealizedLossPositionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7de0-ad0e-06ace8cd7b39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7b75-b003-10a2d2c243db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7de0-ad0e-06ace8cd7b39" xlink:to="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7b75-b003-10a2d2c243db" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-73b5-9c69-01b87ced915a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_019cb57f-d1d9-7b75-b003-10a2d2c243db" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-73b5-9c69-01b87ced915a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-73b5-9c69-01b87ced915a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-73b5-9c69-01b87ced915a" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-73b5-9c69-01b87ced915a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7eea-9d96-ae9c6fb8d2b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cb57f-d1d9-73b5-9c69-01b87ced915a" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7eea-9d96-ae9c6fb8d2b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019cb57f-d1d9-730a-91d2-e23fa03e8841" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7eea-9d96-ae9c6fb8d2b8" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019cb57f-d1d9-730a-91d2-e23fa03e8841" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cb57f-d1d9-74d1-887b-8177469d56c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7eea-9d96-ae9c6fb8d2b8" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cb57f-d1d9-74d1-887b-8177469d56c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019cb57f-d1d9-7129-bf28-10da2861beba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7eea-9d96-ae9c6fb8d2b8" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019cb57f-d1d9-7129-bf28-10da2861beba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralizedDebtObligationsMember_019cb57f-d1d9-7c47-8a4e-694fcbe20cc4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollateralizedDebtObligationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7eea-9d96-ae9c6fb8d2b8" xlink:to="loc_us-gaap_CollateralizedDebtObligationsMember_019cb57f-d1d9-7c47-8a4e-694fcbe20cc4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019cb57f-d1d9-7cff-9608-a54e6351d617" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cb57f-d1d9-7eea-9d96-ae9c6fb8d2b8" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019cb57f-d1d9-7cff-9608-a54e6351d617" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019cb57f-d1d9-724f-a33e-60e63383819d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7de0-ad0e-06ace8cd7b39" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019cb57f-d1d9-724f-a33e-60e63383819d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019cb57f-d1d9-7a77-ab35-ee2d0ae35187" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7de0-ad0e-06ace8cd7b39" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019cb57f-d1d9-7a77-ab35-ee2d0ae35187" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerFairValue_019cb57f-d1d9-7cf0-9035-547221f78c87" xlink:href="fsbc-20260331.xsd#fsbc_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7de0-ad0e-06ace8cd7b39" xlink:to="loc_fsbc_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerFairValue_019cb57f-d1d9-7cf0-9035-547221f78c87" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019cb57f-d1d9-7585-8b86-b6de8d766615" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7de0-ad0e-06ace8cd7b39" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019cb57f-d1d9-7585-8b86-b6de8d766615" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValue_019cb57f-d1d9-7096-9b6e-4f1d44ddf00c" xlink:href="fsbc-20260331.xsd#fsbc_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7de0-ad0e-06ace8cd7b39" xlink:to="loc_fsbc_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValue_019cb57f-d1d9-7096-9b6e-4f1d44ddf00c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019cb57f-d1d9-72a0-871f-eb7a3d4caad9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_019cb57f-d1d9-7de0-ad0e-06ace8cd7b39" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019cb57f-d1d9-72a0-871f-eb7a3d4caad9" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesBalanceofEachMajorProductTypeDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesBalanceofEachMajorProductTypeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesBalanceofEachMajorProductTypeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-77c9-832f-f58b40327132" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d1d9-73da-9f33-bc6848021257" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-77c9-832f-f58b40327132" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d1d9-73da-9f33-bc6848021257" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d1d9-705e-a03f-2808d28b3f97" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d1d9-73da-9f33-bc6848021257" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d1d9-705e-a03f-2808d28b3f97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-705e-a03f-2808d28b3f97_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d1d9-705e-a03f-2808d28b3f97" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-705e-a03f-2808d28b3f97_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d1d9-705e-a03f-2808d28b3f97" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019cb57f-d1d9-7db5-93e5-0331e1dcc27f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:to="loc_us-gaap_CommercialRealEstateMember_019cb57f-d1d9-7db5-93e5-0331e1dcc27f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_019cb57f-d1d9-749d-a326-30c7e0ec40a6" xlink:href="fsbc-20260331.xsd#fsbc_RealEstateCommercialLandAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:to="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_019cb57f-d1d9-749d-a326-30c7e0ec40a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialConstructionLoansMember_019cb57f-d1d9-7655-a16f-d937408200a8" xlink:href="fsbc-20260331.xsd#fsbc_CommercialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:to="loc_fsbc_CommercialConstructionLoansMember_019cb57f-d1d9-7655-a16f-d937408200a8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ResidentialConstructionLoansMember_019cb57f-d1d9-738a-b447-512c0d57d94c" xlink:href="fsbc-20260331.xsd#fsbc_ResidentialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:to="loc_fsbc_ResidentialConstructionLoansMember_019cb57f-d1d9-738a-b447-512c0d57d94c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_019cb57f-d1d9-7a45-8455-c43122987c7b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:to="loc_us-gaap_ResidentialRealEstateMember_019cb57f-d1d9-7a45-8455-c43122987c7b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FarmlandMember_019cb57f-d1d9-7d2a-ade1-1213d60e6960" xlink:href="fsbc-20260331.xsd#fsbc_FarmlandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:to="loc_fsbc_FarmlandMember_019cb57f-d1d9-7d2a-ade1-1213d60e6960" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_019cb57f-d1d9-71fb-b8e1-cde726d42f82" xlink:href="fsbc-20260331.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:to="loc_fsbc_SecuredCommercialLoansMember_019cb57f-d1d9-71fb-b8e1-cde726d42f82" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnsecuredCommercialLoansMember_019cb57f-d1d9-7a59-980e-44230d8fb0c6" xlink:href="fsbc-20260331.xsd#fsbc_UnsecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:to="loc_fsbc_UnsecuredCommercialLoansMember_019cb57f-d1d9-7a59-980e-44230d8fb0c6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_019cb57f-d1d9-7fa8-8bc5-f3ad0df6feff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7475-9e82-cf8a934f2c29" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_019cb57f-d1d9-7fa8-8bc5-f3ad0df6feff" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableBeforeAllowanceForCreditLossAndFee_019cb57f-d1d9-77b0-a8eb-8a228be216cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableBeforeAllowanceForCreditLossAndFee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-77c9-832f-f58b40327132" xlink:to="loc_us-gaap_FinancingReceivableBeforeAllowanceForCreditLossAndFee_019cb57f-d1d9-77b0-a8eb-8a228be216cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_019cb57f-d1d9-7499-93e4-ff292bcd92b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-77c9-832f-f58b40327132" xlink:to="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_019cb57f-d1d9-7499-93e4-ff292bcd92b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableGross_019cb57f-d1d9-7cdc-b9f9-b7a9e35718f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesReceivableGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-77c9-832f-f58b40327132" xlink:to="loc_us-gaap_NotesReceivableGross_019cb57f-d1d9-7cdc-b9f9-b7a9e35718f8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d1d9-76e6-8cd0-942ad5de4421" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-77c9-832f-f58b40327132" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d1d9-76e6-8cd0-942ad5de4421" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableNet_019cb57f-d1d9-7f41-85e2-38cfd6828bba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-77c9-832f-f58b40327132" xlink:to="loc_us-gaap_NotesReceivableNet_019cb57f-d1d9-7f41-85e2-38cfd6828bba" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesCreditQualityIndicatorsRelatedtotheCompanysLoansbyClassDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesCreditQualityIndicatorsRelatedtotheCompanysLoansbyClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesCreditQualityIndicatorsRelatedtotheCompanysLoansbyClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d1d9-7324-81a4-b03396fa47cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d1d9-7324-81a4-b03396fa47cf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d1d9-7b94-a8a6-104eeeab0a7b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d1d9-7324-81a4-b03396fa47cf" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d1d9-7b94-a8a6-104eeeab0a7b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7b94-a8a6-104eeeab0a7b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d1d9-7b94-a8a6-104eeeab0a7b" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7b94-a8a6-104eeeab0a7b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d1d9-7b94-a8a6-104eeeab0a7b" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019cb57f-d1d9-746b-a678-9caee6556a28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:to="loc_us-gaap_CommercialRealEstateMember_019cb57f-d1d9-746b-a678-9caee6556a28" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_019cb57f-d1d9-7bd7-98a4-9a788c14c3cb" xlink:href="fsbc-20260331.xsd#fsbc_RealEstateCommercialLandAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:to="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_019cb57f-d1d9-7bd7-98a4-9a788c14c3cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialConstructionLoansMember_019cb57f-d1d9-704e-ac1c-a1b38ab92d50" xlink:href="fsbc-20260331.xsd#fsbc_CommercialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:to="loc_fsbc_CommercialConstructionLoansMember_019cb57f-d1d9-704e-ac1c-a1b38ab92d50" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ResidentialConstructionLoansMember_019cb57f-d1d9-730a-a34b-09c3a23bd30e" xlink:href="fsbc-20260331.xsd#fsbc_ResidentialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:to="loc_fsbc_ResidentialConstructionLoansMember_019cb57f-d1d9-730a-a34b-09c3a23bd30e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_019cb57f-d1d9-77ce-a956-3b12e1d66d74" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:to="loc_us-gaap_ResidentialRealEstateMember_019cb57f-d1d9-77ce-a956-3b12e1d66d74" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FarmlandMember_019cb57f-d1d9-7893-b172-059257046692" xlink:href="fsbc-20260331.xsd#fsbc_FarmlandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:to="loc_fsbc_FarmlandMember_019cb57f-d1d9-7893-b172-059257046692" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_019d2639-9727-70e8-9b19-aa9c0840a555" xlink:href="fsbc-20260331.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:to="loc_fsbc_SecuredCommercialLoansMember_019d2639-9727-70e8-9b19-aa9c0840a555" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnsecuredCommercialLoansMember_019d263a-ab06-7c2c-99bf-214c417aecf0" xlink:href="fsbc-20260331.xsd#fsbc_UnsecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:to="loc_fsbc_UnsecuredCommercialLoansMember_019d263a-ab06-7c2c-99bf-214c417aecf0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_019cb57f-d1d9-7c5f-9c34-835187f797d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d1d9-7813-94b2-cca0b70aaa5d" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_019cb57f-d1d9-7c5f-9c34-835187f797d2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentAxis_019cb57f-d1d9-7fec-89ce-b0f26346fe68" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InternalCreditAssessmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d1d9-7324-81a4-b03396fa47cf" xlink:to="loc_us-gaap_InternalCreditAssessmentAxis_019cb57f-d1d9-7fec-89ce-b0f26346fe68" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentDomain_019cb57f-d1d9-7fec-89ce-b0f26346fe68_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InternalCreditAssessmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InternalCreditAssessmentAxis_019cb57f-d1d9-7fec-89ce-b0f26346fe68" xlink:to="loc_us-gaap_InternalCreditAssessmentDomain_019cb57f-d1d9-7fec-89ce-b0f26346fe68_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentDomain_019cb57f-d1d9-72ee-9815-5ff47dc278ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InternalCreditAssessmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InternalCreditAssessmentAxis_019cb57f-d1d9-7fec-89ce-b0f26346fe68" xlink:to="loc_us-gaap_InternalCreditAssessmentDomain_019cb57f-d1d9-72ee-9815-5ff47dc278ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PassMember_019cb57f-d1d9-719f-a9bb-e70058df2581" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PassMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019cb57f-d1d9-72ee-9815-5ff47dc278ff" xlink:to="loc_us-gaap_PassMember_019cb57f-d1d9-719f-a9bb-e70058df2581" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_WatchMember_019cb57f-d1d9-7e33-b3b3-2f3605d29ccf" xlink:href="fsbc-20260331.xsd#fsbc_WatchMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019cb57f-d1d9-72ee-9815-5ff47dc278ff" xlink:to="loc_fsbc_WatchMember_019cb57f-d1d9-7e33-b3b3-2f3605d29ccf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubstandardMember_019cb57f-d1d9-7bce-9276-61d65c5a80fc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubstandardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019cb57f-d1d9-72ee-9815-5ff47dc278ff" xlink:to="loc_us-gaap_SubstandardMember_019cb57f-d1d9-7bce-9276-61d65c5a80fc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear_019cb57f-d1d9-7157-9f5d-42e57d2759ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear_019cb57f-d1d9-7157-9f5d-42e57d2759ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear_019cb57f-d1d9-7acd-a092-31e480a804a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear_019cb57f-d1d9-7acd-a092-31e480a804a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear_019cb57f-d1d9-7d45-af2b-1afadecc2d83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear_019cb57f-d1d9-7d45-af2b-1afadecc2d83" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear_019cb57f-d1d9-7c87-8c8d-e99b2bcd621c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear_019cb57f-d1d9-7c87-8c8d-e99b2bcd621c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear_019cb57f-d1d9-7360-b1e2-6b221ab803c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear_019cb57f-d1d9-7360-b1e2-6b221ab803c3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear_019cb57f-d283-7c5e-8731-daa0f6a885fc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear_019cb57f-d283-7c5e-8731-daa0f6a885fc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRevolving_019cb57f-d283-7503-b8b2-051510ea0d90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRevolving"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_FinancingReceivableRevolving_019cb57f-d283-7503-b8b2-051510ea0d90" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRevolvingConvertedToTermLoan_019cb57f-d283-7771-891f-b86083b808c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRevolvingConvertedToTermLoan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_FinancingReceivableRevolvingConvertedToTermLoan_019cb57f-d283-7771-891f-b86083b808c4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableGross_019cb57f-d283-7a86-b69a-d37b08c6ed08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesReceivableGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d1d9-784c-a909-c96c5be651be" xlink:to="loc_us-gaap_NotesReceivableGross_019cb57f-d283-7a86-b69a-d37b08c6ed08" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesGrossChargeoffsbyOriginationYearandClassDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesGrossChargeoffsbyOriginationYearandClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesGrossChargeoffsbyOriginationYearandClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019d665a-e2e7-71f2-b7f2-988c78d73e48" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019d665a-e2e7-71f2-b7f2-988c78d73e48" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d665a-e2e7-7290-97e2-62536ce4d434" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019d665a-e2e7-71f2-b7f2-988c78d73e48" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d665a-e2e7-7290-97e2-62536ce4d434" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d665a-e2e7-7290-97e2-62536ce4d434_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d665a-e2e7-7290-97e2-62536ce4d434" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d665a-e2e7-7290-97e2-62536ce4d434_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d665a-e2e7-726c-bec5-2295b0537426" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d665a-e2e7-7290-97e2-62536ce4d434" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d665a-e2e7-726c-bec5-2295b0537426" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_019d665a-e2e7-77a7-bf67-dee2f71087ac" xlink:href="fsbc-20260331.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d665a-e2e7-726c-bec5-2295b0537426" xlink:to="loc_fsbc_SecuredCommercialLoansMember_019d665a-e2e7-77a7-bf67-dee2f71087ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_019d665a-e2e7-75ab-aa1f-f951b4d7aa17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d665a-e2e7-726c-bec5-2295b0537426" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_019d665a-e2e7-75ab-aa1f-f951b4d7aa17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearOneOriginatedCurrentFiscalYearWriteoff_019d665a-e2e7-713f-be2f-88121866cf53" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearOneOriginatedCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_FinancingReceivableYearOneOriginatedCurrentFiscalYearWriteoff_019d665a-e2e7-713f-be2f-88121866cf53" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-70a9-89a0-52535b56203a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_FinancingReceivableYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-70a9-89a0-52535b56203a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-7d57-8435-ca29f55c1590" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_FinancingReceivableYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-7d57-8435-ca29f55c1590" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-70e9-83a5-49cb6c5b5ed3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_FinancingReceivableYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-70e9-83a5-49cb6c5b5ed3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-75e5-b48d-1642764fa8a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_FinancingReceivableYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-75e5-b48d-1642764fa8a8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-784f-8f68-19ef69130230" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_FinancingReceivableOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff_019d665a-e2e7-784f-8f68-19ef69130230" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRevolvingWriteoff_019d665a-e2e7-729c-9b9d-811f48107e72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRevolvingWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_FinancingReceivableRevolvingWriteoff_019d665a-e2e7-729c-9b9d-811f48107e72" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRevolvingConvertedToTermLoanWriteoff_019d665a-e2e7-7f90-a718-4508bf85db60" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRevolvingConvertedToTermLoanWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_FinancingReceivableRevolvingConvertedToTermLoanWriteoff_019d665a-e2e7-7f90-a718-4508bf85db60" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_019d665a-e2e7-7ab1-877b-559b53edfcf4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d665a-e2e7-72d3-9a66-edb1e1af4e3b" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_019d665a-e2e7-7ab1-877b-559b53edfcf4" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesAgeAnalysisofPastDueLoansbyClassDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesAgeAnalysisofPastDueLoansbyClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesAgeAnalysisofPastDueLoansbyClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-70b4-b1b8-43fa02b4b84a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-78c0-8037-b8e5e20b7682" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-70b4-b1b8-43fa02b4b84a" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-78c0-8037-b8e5e20b7682" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7d4f-a15f-853eba33b8a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-78c0-8037-b8e5e20b7682" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7d4f-a15f-853eba33b8a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7d4f-a15f-853eba33b8a9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7d4f-a15f-853eba33b8a9" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7d4f-a15f-853eba33b8a9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7d4f-a15f-853eba33b8a9" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019cb57f-d283-7e84-a693-84aab8817f18" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:to="loc_us-gaap_CommercialRealEstateMember_019cb57f-d283-7e84-a693-84aab8817f18" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_019cb57f-d283-7c5c-a5fa-cbcc4b7bc362" xlink:href="fsbc-20260331.xsd#fsbc_RealEstateCommercialLandAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:to="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_019cb57f-d283-7c5c-a5fa-cbcc4b7bc362" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialConstructionLoansMember_019cb57f-d283-77a0-8743-09950a74c9e5" xlink:href="fsbc-20260331.xsd#fsbc_CommercialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:to="loc_fsbc_CommercialConstructionLoansMember_019cb57f-d283-77a0-8743-09950a74c9e5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ResidentialConstructionLoansMember_019cb57f-d283-734a-9399-bc176687a053" xlink:href="fsbc-20260331.xsd#fsbc_ResidentialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:to="loc_fsbc_ResidentialConstructionLoansMember_019cb57f-d283-734a-9399-bc176687a053" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_019cb57f-d283-7479-a31a-db5fc589499c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:to="loc_us-gaap_ResidentialRealEstateMember_019cb57f-d283-7479-a31a-db5fc589499c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FarmlandMember_019cb57f-d283-7c15-a9c4-4f9318b0f898" xlink:href="fsbc-20260331.xsd#fsbc_FarmlandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:to="loc_fsbc_FarmlandMember_019cb57f-d283-7c15-a9c4-4f9318b0f898" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_019cb57f-d283-7835-8f2f-03944a70dd3c" xlink:href="fsbc-20260331.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:to="loc_fsbc_SecuredCommercialLoansMember_019cb57f-d283-7835-8f2f-03944a70dd3c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnsecuredCommercialLoansMember_019cb57f-d283-7737-98fc-943fd46b5b4b" xlink:href="fsbc-20260331.xsd#fsbc_UnsecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:to="loc_fsbc_UnsecuredCommercialLoansMember_019cb57f-d283-7737-98fc-943fd46b5b4b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_019cb57f-d283-7c79-afd9-b7c3cdda04f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7dd6-bf09-fe067d3c91c3" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_019cb57f-d283-7c79-afd9-b7c3cdda04f0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019cb57f-d283-72b3-9f51-d3d2e3bd5068" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesPeriodPastDueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-78c0-8037-b8e5e20b7682" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019cb57f-d283-72b3-9f51-d3d2e3bd5068" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019cb57f-d283-72b3-9f51-d3d2e3bd5068_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019cb57f-d283-72b3-9f51-d3d2e3bd5068" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019cb57f-d283-72b3-9f51-d3d2e3bd5068_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019cb57f-d283-743d-9105-4016607581a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019cb57f-d283-72b3-9f51-d3d2e3bd5068" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019cb57f-d283-743d-9105-4016607581a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivables30To59DaysPastDueMember_019cb57f-d283-7ef4-b406-3bbb2bdfc479" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivables30To59DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019cb57f-d283-743d-9105-4016607581a5" xlink:to="loc_us-gaap_FinancingReceivables30To59DaysPastDueMember_019cb57f-d283-7ef4-b406-3bbb2bdfc479" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivables60To89DaysPastDueMember_019cb57f-d283-7974-838f-47e26bf7abd7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivables60To89DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019cb57f-d283-743d-9105-4016607581a5" xlink:to="loc_us-gaap_FinancingReceivables60To89DaysPastDueMember_019cb57f-d283-7974-838f-47e26bf7abd7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_019cb57f-d283-7d0d-8d12-93c9aca58f41" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019cb57f-d283-743d-9105-4016607581a5" xlink:to="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_019cb57f-d283-7d0d-8d12-93c9aca58f41" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialAssetPastDueMember_019cb57f-d283-7ff8-97c5-0ffc538fc23f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialAssetPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019cb57f-d283-743d-9105-4016607581a5" xlink:to="loc_us-gaap_FinancialAssetPastDueMember_019cb57f-d283-7ff8-97c5-0ffc538fc23f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialAssetNotPastDueMember_019cb57f-d283-7031-9974-af3a7c3ac7dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialAssetNotPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019cb57f-d283-743d-9105-4016607581a5" xlink:to="loc_us-gaap_FinancialAssetNotPastDueMember_019cb57f-d283-7031-9974-af3a7c3ac7dd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesReceivableGross_019cb57f-d283-70b7-8604-a95483dc0fdc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesReceivableGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-70b4-b1b8-43fa02b4b84a" xlink:to="loc_us-gaap_NotesReceivableGross_019cb57f-d283-70b7-8604-a95483dc0fdc" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-774c-8403-61921ff1dbad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7337-b4f7-a1d501bb0988" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-774c-8403-61921ff1dbad" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7337-b4f7-a1d501bb0988" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankBranchAxis_019cb57f-d283-794c-be78-c26917531fe7" xlink:href="fsbc-20260331.xsd#fsbc_FederalReserveBankBranchAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7337-b4f7-a1d501bb0988" xlink:to="loc_fsbc_FederalReserveBankBranchAxis_019cb57f-d283-794c-be78-c26917531fe7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankBranchDomain_019cb57f-d283-794c-be78-c26917531fe7_default" xlink:href="fsbc-20260331.xsd#fsbc_FederalReserveBankBranchDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_fsbc_FederalReserveBankBranchAxis_019cb57f-d283-794c-be78-c26917531fe7" xlink:to="loc_fsbc_FederalReserveBankBranchDomain_019cb57f-d283-794c-be78-c26917531fe7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankBranchDomain_019cb57f-d283-71fb-97f0-6ddfd38d20dd" xlink:href="fsbc-20260331.xsd#fsbc_FederalReserveBankBranchDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_fsbc_FederalReserveBankBranchAxis_019cb57f-d283-794c-be78-c26917531fe7" xlink:to="loc_fsbc_FederalReserveBankBranchDomain_019cb57f-d283-71fb-97f0-6ddfd38d20dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankOfSanFranciscoMember_019cb57f-d283-7d63-be0a-7a15e6322ecf" xlink:href="fsbc-20260331.xsd#fsbc_FederalReserveBankOfSanFranciscoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_FederalReserveBankBranchDomain_019cb57f-d283-71fb-97f0-6ddfd38d20dd" xlink:to="loc_fsbc_FederalReserveBankOfSanFranciscoMember_019cb57f-d283-7d63-be0a-7a15e6322ecf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7a33-820e-fbd5416cb1ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7337-b4f7-a1d501bb0988" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7a33-820e-fbd5416cb1ce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7a33-820e-fbd5416cb1ce_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7a33-820e-fbd5416cb1ce" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7a33-820e-fbd5416cb1ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-77a6-bb1e-55f5d1a05f88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7a33-820e-fbd5416cb1ce" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-77a6-bb1e-55f5d1a05f88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CreditBalanceMember_019df167-a194-70af-97bd-b8584a6a2ae6" xlink:href="fsbc-20260331.xsd#fsbc_CreditBalanceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-77a6-bb1e-55f5d1a05f88" xlink:to="loc_fsbc_CreditBalanceMember_019df167-a194-70af-97bd-b8584a6a2ae6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing_019cb57f-d283-778f-9547-b77911ea4ecd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-774c-8403-61921ff1dbad" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing_019cb57f-d283-778f-9547-b77911ea4ecd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageLoansInProcessOfForeclosureAmount_019cb57f-d283-73e5-ba16-4c2f84e0b2de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageLoansInProcessOfForeclosureAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-774c-8403-61921ff1dbad" xlink:to="loc_us-gaap_MortgageLoansInProcessOfForeclosureAmount_019cb57f-d283-73e5-ba16-4c2f84e0b2de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableNonaccrualInterestIncome_019dccaf-914b-7a63-ae77-a143ffd41b29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableNonaccrualInterestIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-774c-8403-61921ff1dbad" xlink:to="loc_us-gaap_FinancingReceivableNonaccrualInterestIncome_019dccaf-914b-7a63-ae77-a143ffd41b29" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FinancingReceivableNonaccrualAllowanceForCreditLoss_019dccaf-914b-7d16-bc45-7c1d2ba3a6b8" xlink:href="fsbc-20260331.xsd#fsbc_FinancingReceivableNonaccrualAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-774c-8403-61921ff1dbad" xlink:to="loc_fsbc_FinancingReceivableNonaccrualAllowanceForCreditLoss_019dccaf-914b-7d16-bc45-7c1d2ba3a6b8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FinancingReceivableNonaccrualInterestIncomeForegone_019cb57f-d283-7c82-bc67-ca835e37ee7b" xlink:href="fsbc-20260331.xsd#fsbc_FinancingReceivableNonaccrualInterestIncomeForegone"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-774c-8403-61921ff1dbad" xlink:to="loc_fsbc_FinancingReceivableNonaccrualInterestIncomeForegone_019cb57f-d283-7c82-bc67-ca835e37ee7b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1_019cb57f-d283-70b1-8012-03612afbac99" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-774c-8403-61921ff1dbad" xlink:to="loc_us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1_019cb57f-d283-70b1-8012-03612afbac99" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged_019cb57f-d283-7013-99db-d407a6efc44d" xlink:href="fsbc-20260331.xsd#fsbc_FederalReserveBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-774c-8403-61921ff1dbad" xlink:to="loc_fsbc_FederalReserveBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged_019cb57f-d283-7013-99db-d407a6efc44d" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesCollateralDependentLoansbyClassDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesCollateralDependentLoansbyClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesCollateralDependentLoansbyClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d3cd7-fa1a-7630-8bfd-baffe4f320cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019d3cd7-fa1a-7a45-acfb-c2a227fcea24" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d3cd7-fa1a-7630-8bfd-baffe4f320cc" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019d3cd7-fa1a-7a45-acfb-c2a227fcea24" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d3cd7-fa1a-7e20-9d64-847d4d47acb1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019d3cd7-fa1a-7a45-acfb-c2a227fcea24" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d3cd7-fa1a-7e20-9d64-847d4d47acb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cd7-fa1a-7e20-9d64-847d4d47acb1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d3cd7-fa1a-7e20-9d64-847d4d47acb1" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cd7-fa1a-7e20-9d64-847d4d47acb1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cd7-fa1a-777a-aa98-3bd47fac6416" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d3cd7-fa1a-7e20-9d64-847d4d47acb1" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cd7-fa1a-777a-aa98-3bd47fac6416" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019d3cd7-fa1a-7164-b7ed-1605efde3fb4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cd7-fa1a-777a-aa98-3bd47fac6416" xlink:to="loc_us-gaap_CommercialRealEstateMember_019d3cd7-fa1a-7164-b7ed-1605efde3fb4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_019d3cd7-fa1a-789f-9754-e60e3c87efa4" xlink:href="fsbc-20260331.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cd7-fa1a-777a-aa98-3bd47fac6416" xlink:to="loc_fsbc_SecuredCommercialLoansMember_019d3cd7-fa1a-789f-9754-e60e3c87efa4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment_019d3cd7-fa1a-7dc7-92fb-eb0a024e90c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d3cd7-fa1a-7630-8bfd-baffe4f320cc" xlink:to="loc_us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment_019d3cd7-fa1a-7dc7-92fb-eb0a024e90c0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesLoansModifiedduetoFinancialDistressDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesLoansModifiedduetoFinancialDistressDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesLoansModifiedduetoFinancialDistressDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d6660-069b-79b1-8b38-7d8719d780d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019d6660-069b-7e40-90ee-c172eeac86ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d6660-069b-79b1-8b38-7d8719d780d7" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019d6660-069b-7e40-90ee-c172eeac86ee" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d6660-069b-7de6-8c10-9d491fbc6fa1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019d6660-069b-7e40-90ee-c172eeac86ee" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d6660-069b-7de6-8c10-9d491fbc6fa1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d6660-069b-7de6-8c10-9d491fbc6fa1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d6660-069b-7de6-8c10-9d491fbc6fa1" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d6660-069b-7de6-8c10-9d491fbc6fa1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d6660-069b-7b21-acd9-e66bdc54b84f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d6660-069b-7de6-8c10-9d491fbc6fa1" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d6660-069b-7b21-acd9-e66bdc54b84f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019d6660-069b-73a7-a8ca-7fc12dd131a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d6660-069b-7b21-acd9-e66bdc54b84f" xlink:to="loc_us-gaap_CommercialRealEstateMember_019d6660-069b-73a7-a8ca-7fc12dd131a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoanRestructuringModificationAxis_019d6660-069b-7ce9-9fa2-06616791a876" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoanRestructuringModificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019d6660-069b-7e40-90ee-c172eeac86ee" xlink:to="loc_us-gaap_LoanRestructuringModificationAxis_019d6660-069b-7ce9-9fa2-06616791a876" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoanRestructuringModificationDomain_019d6660-069b-7ce9-9fa2-06616791a876_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoanRestructuringModificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LoanRestructuringModificationAxis_019d6660-069b-7ce9-9fa2-06616791a876" xlink:to="loc_us-gaap_LoanRestructuringModificationDomain_019d6660-069b-7ce9-9fa2-06616791a876_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoanRestructuringModificationDomain_019d6660-069b-7ec8-8602-cda664a930f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoanRestructuringModificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LoanRestructuringModificationAxis_019d6660-069b-7ce9-9fa2-06616791a876" xlink:to="loc_us-gaap_LoanRestructuringModificationDomain_019d6660-069b-7ec8-8602-cda664a930f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentDeferralMember_019d6660-069b-751f-8164-1f56a2ec2b44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentDeferralMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoanRestructuringModificationDomain_019d6660-069b-7ec8-8602-cda664a930f8" xlink:to="loc_us-gaap_PaymentDeferralMember_019d6660-069b-751f-8164-1f56a2ec2b44" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2_019d6660-069b-70a6-9f0f-a41bc20ec797" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d6660-069b-79b1-8b38-7d8719d780d7" xlink:to="loc_us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2_019d6660-069b-70a6-9f0f-a41bc20ec797" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesPercent_019d6660-069b-73bf-8e0c-a49a9694321d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019d6660-069b-79b1-8b38-7d8719d780d7" xlink:to="loc_us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesPercent_019d6660-069b-73bf-8e0c-a49a9694321d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesFinancialEffectofLoansModifiedduetoFinancialDistressDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesFinancialEffectofLoansModifiedduetoFinancialDistressDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesFinancialEffectofLoansModifiedduetoFinancialDistressDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModificationsLineItems_019d3cdb-e186-7077-9403-71d2f09f1132" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableModificationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019d3cdb-e186-7dfb-880d-4a7e24a643cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019d3cdb-e186-7077-9403-71d2f09f1132" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019d3cdb-e186-7dfb-880d-4a7e24a643cc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d3cdb-e186-7f2a-ae9b-47e468c244ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019d3cdb-e186-7dfb-880d-4a7e24a643cc" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d3cdb-e186-7f2a-ae9b-47e468c244ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cdb-e186-7f2a-ae9b-47e468c244ad_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d3cdb-e186-7f2a-ae9b-47e468c244ad" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cdb-e186-7f2a-ae9b-47e468c244ad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cdb-e186-7771-86c0-fe0b7555ac83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019d3cdb-e186-7f2a-ae9b-47e468c244ad" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cdb-e186-7771-86c0-fe0b7555ac83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019d3cdb-e186-7199-9ced-14999b5869b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019d3cdb-e186-7771-86c0-fe0b7555ac83" xlink:to="loc_us-gaap_CommercialRealEstateMember_019d3cdb-e186-7199-9ced-14999b5869b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FinancingReceivableModifiedInPeriodReducedLoanPaymentByBorrowerPeriod_019d3cdb-e186-70d1-809f-055d78e89291" xlink:href="fsbc-20260331.xsd#fsbc_FinancingReceivableModifiedInPeriodReducedLoanPaymentByBorrowerPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019d3cdb-e186-7077-9403-71d2f09f1132" xlink:to="loc_fsbc_FinancingReceivableModifiedInPeriodReducedLoanPaymentByBorrowerPeriod_019d3cdb-e186-70d1-809f-055d78e89291" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesNonaccrualLoansSegregatedbyClassDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesNonaccrualLoansSegregatedbyClassDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesNonaccrualLoansSegregatedbyClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-7539-9965-7e15ec103709" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7f27-a5f2-f5dfad7b7ee3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-7539-9965-7e15ec103709" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7f27-a5f2-f5dfad7b7ee3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7ef3-847c-746f5ba22409" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7f27-a5f2-f5dfad7b7ee3" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7ef3-847c-746f5ba22409" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7ef3-847c-746f5ba22409_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7ef3-847c-746f5ba22409" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7ef3-847c-746f5ba22409_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7691-abc6-299b9eb4aab1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-7ef3-847c-746f5ba22409" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7691-abc6-299b9eb4aab1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019cb57f-d283-7dbc-9b15-9d82ace27ff9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7691-abc6-299b9eb4aab1" xlink:to="loc_us-gaap_CommercialRealEstateMember_019cb57f-d283-7dbc-9b15-9d82ace27ff9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_019cb57f-d283-7ff3-8070-86d7b0816d50" xlink:href="fsbc-20260331.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7691-abc6-299b9eb4aab1" xlink:to="loc_fsbc_SecuredCommercialLoansMember_019cb57f-d283-7ff3-8070-86d7b0816d50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus_019cb57f-d283-7340-8926-bf0cdc4bfc86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-7539-9965-7e15ec103709" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus_019cb57f-d283-7340-8926-bf0cdc4bfc86" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesAllowanceforCreditLossesDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesAllowanceforCreditLossesDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesAllowanceforCreditLossesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-7f6f-8897-1ed0f3d7919b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7b4f-98cd-4a8848e35178" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-7f6f-8897-1ed0f3d7919b" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7b4f-98cd-4a8848e35178" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-764b-8513-940330f39e21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-7b4f-98cd-4a8848e35178" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-764b-8513-940330f39e21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-764b-8513-940330f39e21_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-764b-8513-940330f39e21" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-764b-8513-940330f39e21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-764b-8513-940330f39e21" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019cb57f-d283-7314-9677-a02e1b03979c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:to="loc_us-gaap_CommercialRealEstateMember_019cb57f-d283-7314-9677-a02e1b03979c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_019cb57f-d283-7870-bb47-305d0f23c494" xlink:href="fsbc-20260331.xsd#fsbc_RealEstateCommercialLandAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:to="loc_fsbc_RealEstateCommercialLandAndDevelopmentMember_019cb57f-d283-7870-bb47-305d0f23c494" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialConstructionLoansMember_019cb57f-d283-751f-9398-acfd38213df4" xlink:href="fsbc-20260331.xsd#fsbc_CommercialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:to="loc_fsbc_CommercialConstructionLoansMember_019cb57f-d283-751f-9398-acfd38213df4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ResidentialConstructionLoansMember_019cb57f-d283-7014-bb4a-e914dac25480" xlink:href="fsbc-20260331.xsd#fsbc_ResidentialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:to="loc_fsbc_ResidentialConstructionLoansMember_019cb57f-d283-7014-bb4a-e914dac25480" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_019cb57f-d283-700c-a299-3aadf56cc1b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:to="loc_us-gaap_ResidentialRealEstateMember_019cb57f-d283-700c-a299-3aadf56cc1b9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FarmlandMember_019cb57f-d283-7d3e-a587-f3403d7ff89a" xlink:href="fsbc-20260331.xsd#fsbc_FarmlandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:to="loc_fsbc_FarmlandMember_019cb57f-d283-7d3e-a587-f3403d7ff89a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecuredCommercialLoansMember_019cb57f-d283-72d4-9f86-a43526b172b7" xlink:href="fsbc-20260331.xsd#fsbc_SecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:to="loc_fsbc_SecuredCommercialLoansMember_019cb57f-d283-72d4-9f86-a43526b172b7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnsecuredCommercialLoansMember_019cb57f-d283-7513-b20e-784a4a901936" xlink:href="fsbc-20260331.xsd#fsbc_UnsecuredCommercialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:to="loc_fsbc_UnsecuredCommercialLoansMember_019cb57f-d283-7513-b20e-784a4a901936" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_019cb57f-d283-71dd-a845-ecb788560647" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7c1a-a8a8-f087a13d4ddf" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_019cb57f-d283-71dd-a845-ecb788560647" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-7dcc-859e-93c15983ddc6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-7f6f-8897-1ed0f3d7919b" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-7dcc-859e-93c15983ddc6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d283-7fff-bd6e-eb259eaeb1a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-7dcc-859e-93c15983ddc6" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d283-7fff-bd6e-eb259eaeb1a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_019cb57f-d283-71e6-a409-5de3b34e7039" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-7dcc-859e-93c15983ddc6" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_019cb57f-d283-71e6-a409-5de3b34e7039" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery_019cb57f-d283-7039-a963-b1a8b573bd2c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-7dcc-859e-93c15983ddc6" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery_019cb57f-d283-7039-a963-b1a8b573bd2c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForLoanLossesExpensed_019cb57f-d283-7433-8e15-c8f5c4a5303a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForLoanLossesExpensed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-7dcc-859e-93c15983ddc6" xlink:to="loc_us-gaap_ProvisionForLoanLossesExpensed_019cb57f-d283-7433-8e15-c8f5c4a5303a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d283-761a-a2c0-294e07216baa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesUnfundedLoanCommitmentReserveDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#LoansandAllowanceforCreditLossesUnfundedLoanCommitmentReserveDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/LoansandAllowanceforCreditLossesUnfundedLoanCommitmentReserveDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-7f5d-8846-b959b233b853" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-71d4-80dc-f8bc0a89c0d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-7f5d-8846-b959b233b853" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-71d4-80dc-f8bc0a89c0d5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-79c3-a056-1e8eef858465" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019cb57f-d283-71d4-80dc-f8bc0a89c0d5" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-79c3-a056-1e8eef858465" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-79c3-a056-1e8eef858465_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-79c3-a056-1e8eef858465" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-79c3-a056-1e8eef858465_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7e76-9bbe-ee9e5e3b5994" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-79c3-a056-1e8eef858465" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7e76-9bbe-ee9e5e3b5994" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnfundedLoanCommitmentMember_019cb57f-d283-7bb9-8701-41ef03c2a6ba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnfundedLoanCommitmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-7e76-9bbe-ee9e5e3b5994" xlink:to="loc_us-gaap_UnfundedLoanCommitmentMember_019cb57f-d283-7bb9-8701-41ef03c2a6ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-75cf-8f8b-f8e18777ba64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019cb57f-d283-7f5d-8846-b959b233b853" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-75cf-8f8b-f8e18777ba64" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d283-7e53-9621-215640646da4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-75cf-8f8b-f8e18777ba64" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d283-7e53-9621-215640646da4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForLoanLossesExpensed_019cb57f-d283-7bca-8b27-12625b916dec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForLoanLossesExpensed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward_019cb57f-d283-75cf-8f8b-f8e18777ba64" xlink:to="loc_us-gaap_ProvisionForLoanLossesExpensed_019cb57f-d283-7bca-8b27-12625b916dec" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d283-7982-8b93-4a06e4c07d08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/InterestBearingDepositsCompositionofNetworkDepositsDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#InterestBearingDepositsCompositionofNetworkDepositsDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/InterestBearingDepositsCompositionofNetworkDepositsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_InterestBearingDepositsLineItems_019cb57f-d283-7337-9668-3dfdb9c03731" xlink:href="fsbc-20260331.xsd#fsbc_InterestBearingDepositsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_InterestBearingDepositsTable_019cb57f-d283-7354-ae21-db99b1fce62a" xlink:href="fsbc-20260331.xsd#fsbc_InterestBearingDepositsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_fsbc_InterestBearingDepositsLineItems_019cb57f-d283-7337-9668-3dfdb9c03731" xlink:to="loc_fsbc_InterestBearingDepositsTable_019cb57f-d283-7354-ae21-db99b1fce62a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019cb57f-d283-7ae4-a9cb-c89f07278c58" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_fsbc_InterestBearingDepositsTable_019cb57f-d283-7354-ae21-db99b1fce62a" xlink:to="loc_srt_ProductOrServiceAxis_019cb57f-d283-7ae4-a9cb-c89f07278c58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019cb57f-d283-7ae4-a9cb-c89f07278c58_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019cb57f-d283-7ae4-a9cb-c89f07278c58" xlink:to="loc_srt_ProductsAndServicesDomain_019cb57f-d283-7ae4-a9cb-c89f07278c58_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019cb57f-d283-754c-a5e9-c5f7eba9688d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019cb57f-d283-7ae4-a9cb-c89f07278c58" xlink:to="loc_srt_ProductsAndServicesDomain_019cb57f-d283-754c-a5e9-c5f7eba9688d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CDARSMember_019cb57f-d283-7e43-aa7f-2733b7210c0a" xlink:href="fsbc-20260331.xsd#fsbc_CDARSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019cb57f-d283-754c-a5e9-c5f7eba9688d" xlink:to="loc_fsbc_CDARSMember_019cb57f-d283-7e43-aa7f-2733b7210c0a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ICSMember_019cb57f-d283-7659-a135-1c16fc8f52d0" xlink:href="fsbc-20260331.xsd#fsbc_ICSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019cb57f-d283-754c-a5e9-c5f7eba9688d" xlink:to="loc_fsbc_ICSMember_019cb57f-d283-7659-a135-1c16fc8f52d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_NetworkDeposits_019cb57f-d283-71e4-9c69-2b4425a2b40a" xlink:href="fsbc-20260331.xsd#fsbc_NetworkDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_InterestBearingDepositsLineItems_019cb57f-d283-7337-9668-3dfdb9c03731" xlink:to="loc_fsbc_NetworkDeposits_019cb57f-d283-71e4-9c69-2b4425a2b40a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/SubordinatedNotesandOtherBorrowingsNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#SubordinatedNotesandOtherBorrowingsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/SubordinatedNotesandOtherBorrowingsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019cb57f-d283-7910-8693-be2330bc6d14" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_DebtInstrumentTable_019cb57f-d283-7910-8693-be2330bc6d14" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019cb57f-d283-7143-8361-c080101436f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019cb57f-d283-7910-8693-be2330bc6d14" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019cb57f-d283-7143-8361-c080101436f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7143-8361-c080101436f4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019cb57f-d283-7143-8361-c080101436f4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7143-8361-c080101436f4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7771-a7d4-ee01311e4ea0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019cb57f-d283-7143-8361-c080101436f4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7771-a7d4-ee01311e4ea0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebtMember_019cb57f-d283-785a-91c7-b7445a0ab788" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7771-a7d4-ee01311e4ea0" xlink:to="loc_us-gaap_SubordinatedDebtMember_019cb57f-d283-785a-91c7-b7445a0ab788" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019cb57f-d283-78c7-a172-8a500f941fbd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7771-a7d4-ee01311e4ea0" xlink:to="loc_us-gaap_LineOfCreditMember_019cb57f-d283-78c7-a172-8a500f941fbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019cb57f-d283-72cd-a727-8c464dd892c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019cb57f-d283-7910-8693-be2330bc6d14" xlink:to="loc_us-gaap_DebtInstrumentAxis_019cb57f-d283-72cd-a727-8c464dd892c4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-72cd-a727-8c464dd892c4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cb57f-d283-72cd-a727-8c464dd892c4" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-72cd-a727-8c464dd892c4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7184-81b5-2baf9fc62d15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cb57f-d283-72cd-a727-8c464dd892c4" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7184-81b5-2baf9fc62d15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FixedToFloatingRateSubordinatedNotesIssuedAugust2022Member_019cb57f-d283-7772-b1a7-8a7f82056e32" xlink:href="fsbc-20260331.xsd#fsbc_FixedToFloatingRateSubordinatedNotesIssuedAugust2022Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7184-81b5-2baf9fc62d15" xlink:to="loc_fsbc_FixedToFloatingRateSubordinatedNotesIssuedAugust2022Member_019cb57f-d283-7772-b1a7-8a7f82056e32" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_EntityControlledByExistingAndFormerBoardMembersAndAffiliatesMember_019cb57f-d283-7415-b509-2a7da0dcdef8" xlink:href="fsbc-20260331.xsd#fsbc_EntityControlledByExistingAndFormerBoardMembersAndAffiliatesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7184-81b5-2baf9fc62d15" xlink:to="loc_fsbc_EntityControlledByExistingAndFormerBoardMembersAndAffiliatesMember_019cb57f-d283-7415-b509-2a7da0dcdef8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FederalReserveDiscountWindowMember_019cb57f-d283-71ec-bed9-dba85f45ede2" xlink:href="fsbc-20260331.xsd#fsbc_FederalReserveDiscountWindowMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7184-81b5-2baf9fc62d15" xlink:to="loc_fsbc_FederalReserveDiscountWindowMember_019cb57f-d283-71ec-bed9-dba85f45ede2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FiveUnsecuredFederalFundsLineOfCreditMember_019cb57f-d283-72fc-8222-70beb18e9a44" xlink:href="fsbc-20260331.xsd#fsbc_FiveUnsecuredFederalFundsLineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7184-81b5-2baf9fc62d15" xlink:to="loc_fsbc_FiveUnsecuredFederalFundsLineOfCreditMember_019cb57f-d283-72fc-8222-70beb18e9a44" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019cb57f-d283-78bd-8a39-ccca96bb8deb" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019cb57f-d283-7910-8693-be2330bc6d14" xlink:to="loc_srt_RangeAxis_019cb57f-d283-78bd-8a39-ccca96bb8deb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019cb57f-d283-78bd-8a39-ccca96bb8deb_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019cb57f-d283-78bd-8a39-ccca96bb8deb" xlink:to="loc_srt_RangeMember_019cb57f-d283-78bd-8a39-ccca96bb8deb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019cb57f-d283-7810-be3a-a44c0517f9a0" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019cb57f-d283-78bd-8a39-ccca96bb8deb" xlink:to="loc_srt_RangeMember_019cb57f-d283-7810-be3a-a44c0517f9a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019cb57f-d283-740d-b70d-320aa622855d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019cb57f-d283-7810-be3a-a44c0517f9a0" xlink:to="loc_srt_MinimumMember_019cb57f-d283-740d-b70d-320aa622855d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019cb57f-d283-7490-abb9-f9a6044239a3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019cb57f-d283-7810-be3a-a44c0517f9a0" xlink:to="loc_srt_MaximumMember_019cb57f-d283-7490-abb9-f9a6044239a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebt_019cb57f-d283-76f1-892c-408e00f5f16d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_SubordinatedDebt_019cb57f-d283-76f1-892c-408e00f5f16d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019cb57f-d283-7b8a-bf73-b1ba2dc05e7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019cb57f-d283-7b8a-bf73-b1ba2dc05e7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedBorrowingInterestRate_019cb57f-d283-7b51-bc7f-d410d759faa6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedBorrowingInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_SubordinatedBorrowingInterestRate_019cb57f-d283-7b51-bc7f-d410d759faa6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019cb57f-d283-7a22-a829-97868a75e454" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019cb57f-d283-7a22-a829-97868a75e454" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_DebtInstrumentFloatingInterestRate_019cb57f-d283-7be9-bd1b-8e754ade7a9d" xlink:href="fsbc-20260331.xsd#fsbc_DebtInstrumentFloatingInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_fsbc_DebtInstrumentFloatingInterestRate_019cb57f-d283-7be9-bd1b-8e754ade7a9d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019cb57f-d283-70b3-9f76-69349fb29e80" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019cb57f-d283-70b3-9f76-69349fb29e80" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedAmortizationDeferredFinanceCosts_019cb57f-d283-7352-a381-e3cd055f8083" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedAmortizationDeferredFinanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_AccumulatedAmortizationDeferredFinanceCosts_019cb57f-d283-7352-a381-e3cd055f8083" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019cb57f-d283-7f8e-9556-f84f38a9f7aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019cb57f-d283-7f8e-9556-f84f38a9f7aa" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019cb57f-d283-7936-89d1-dd0548c5464a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019cb57f-d283-7936-89d1-dd0548c5464a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_019cb57f-d283-7aaf-9f81-c7e777297d3e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_019cb57f-d283-7aaf-9f81-c7e777297d3e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_NumberOfUnsecuredFederalFunds_019cb57f-d283-71cc-bfc0-29d366c1f1cb" xlink:href="fsbc-20260331.xsd#fsbc_NumberOfUnsecuredFederalFunds"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_fsbc_NumberOfUnsecuredFederalFunds_019cb57f-d283-71cc-bfc0-29d366c1f1cb" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_NumberOfCorrespondentBanks_019cb57f-d283-7e5e-8f4d-7f36d8b103d5" xlink:href="fsbc-20260331.xsd#fsbc_NumberOfCorrespondentBanks"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_fsbc_NumberOfCorrespondentBanks_019cb57f-d283-7e5e-8f4d-7f36d8b103d5" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_019cb57f-d283-7596-a427-b45113be9c42" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7103-9ec8-de5a6ea62ee0" xlink:to="loc_us-gaap_LineOfCredit_019cb57f-d283-7596-a427-b45113be9c42" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/SubordinatedNotesandOtherBorrowingsScheduleofBorrowingCapacityDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#SubordinatedNotesandOtherBorrowingsScheduleofBorrowingCapacityDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/SubordinatedNotesandOtherBorrowingsScheduleofBorrowingCapacityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7627-9e36-7fa7f2f1c370" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019cb57f-d283-7994-94b4-c573ca0b4e4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7627-9e36-7fa7f2f1c370" xlink:to="loc_us-gaap_DebtInstrumentTable_019cb57f-d283-7994-94b4-c573ca0b4e4a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019cb57f-d283-7824-9fe5-3dcb4599d067" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019cb57f-d283-7994-94b4-c573ca0b4e4a" xlink:to="loc_us-gaap_DebtInstrumentAxis_019cb57f-d283-7824-9fe5-3dcb4599d067" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7824-9fe5-3dcb4599d067_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cb57f-d283-7824-9fe5-3dcb4599d067" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7824-9fe5-3dcb4599d067_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7da1-873a-8e28c6592d2f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cb57f-d283-7824-9fe5-3dcb4599d067" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7da1-873a-8e28c6592d2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_FHLBAgreementMember_019cb57f-d283-7439-a6cc-d040bd1b81b8" xlink:href="fsbc-20260331.xsd#fsbc_FHLBAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7da1-873a-8e28c6592d2f" xlink:to="loc_fsbc_FHLBAgreementMember_019cb57f-d283-7439-a6cc-d040bd1b81b8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_LettersOfCreditPledgedAsSecurityForStateOfCaliforniaDepositsMember_019cb57f-d283-7ddc-8ec7-cd57363a0971" xlink:href="fsbc-20260331.xsd#fsbc_LettersOfCreditPledgedAsSecurityForStateOfCaliforniaDepositsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7da1-873a-8e28c6592d2f" xlink:to="loc_fsbc_LettersOfCreditPledgedAsSecurityForStateOfCaliforniaDepositsMember_019cb57f-d283-7ddc-8ec7-cd57363a0971" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_SecureLocalAgencyDepositMember_019cb57f-d283-7792-a539-8edc1a602a9d" xlink:href="fsbc-20260331.xsd#fsbc_SecureLocalAgencyDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cb57f-d283-7da1-873a-8e28c6592d2f" xlink:to="loc_fsbc_SecureLocalAgencyDepositMember_019cb57f-d283-7792-a539-8edc1a602a9d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019cb57f-d283-7988-8274-6c8aa22453fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019cb57f-d283-7994-94b4-c573ca0b4e4a" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019cb57f-d283-7988-8274-6c8aa22453fb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7988-8274-6c8aa22453fb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019cb57f-d283-7988-8274-6c8aa22453fb" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7988-8274-6c8aa22453fb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7e55-b03a-f48dd38af88f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019cb57f-d283-7988-8274-6c8aa22453fb" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7e55-b03a-f48dd38af88f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019cb57f-d283-743e-973b-b8275b838189" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7e55-b03a-f48dd38af88f" xlink:to="loc_us-gaap_LineOfCreditMember_019cb57f-d283-743e-973b-b8275b838189" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_019cb57f-d283-73a3-af6d-e01ce99fcd6f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019cb57f-d283-7e55-b03a-f48dd38af88f" xlink:to="loc_us-gaap_LetterOfCreditMember_019cb57f-d283-73a3-af6d-e01ce99fcd6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019cb57f-d283-72c8-a45e-41b56bfdddc7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7627-9e36-7fa7f2f1c370" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019cb57f-d283-72c8-a45e-41b56bfdddc7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_019cb57f-d283-75c3-a8e8-b14046244e51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7627-9e36-7fa7f2f1c370" xlink:to="loc_us-gaap_LineOfCredit_019cb57f-d283-75c3-a8e8-b14046244e51" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_019cb57f-d283-7070-bb52-827c2371652b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7627-9e36-7fa7f2f1c370" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_019cb57f-d283-7070-bb52-827c2371652b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019cb57f-d283-73c6-9cd9-8f83495c396f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cb57f-d283-7627-9e36-7fa7f2f1c370" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019cb57f-d283-73c6-9cd9-8f83495c396f" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/ShareholdersEquityNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#ShareholdersEquityNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/ShareholdersEquityNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7b29-95e7-9f5020eece87" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7b29-95e7-9f5020eece87" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusAxis_019cb57f-d283-7475-a99b-6eaaa7e6548a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GranteeStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7b29-95e7-9f5020eece87" xlink:to="loc_us-gaap_GranteeStatusAxis_019cb57f-d283-7475-a99b-6eaaa7e6548a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_019cb57f-d283-7475-a99b-6eaaa7e6548a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GranteeStatusAxis_019cb57f-d283-7475-a99b-6eaaa7e6548a" xlink:to="loc_us-gaap_GranteeStatusDomain_019cb57f-d283-7475-a99b-6eaaa7e6548a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_019cb57f-d283-75b4-be9a-6350d0f508c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GranteeStatusAxis_019cb57f-d283-7475-a99b-6eaaa7e6548a" xlink:to="loc_us-gaap_GranteeStatusDomain_019cb57f-d283-75b4-be9a-6350d0f508c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedPaymentArrangementExecutivesAndDirectorsMember_019cb57f-d283-7a79-87f9-411e2695906f" xlink:href="fsbc-20260331.xsd#fsbc_ShareBasedPaymentArrangementExecutivesAndDirectorsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019cb57f-d283-75b4-be9a-6350d0f508c6" xlink:to="loc_fsbc_ShareBasedPaymentArrangementExecutivesAndDirectorsMember_019cb57f-d283-7a79-87f9-411e2695906f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_019cb57f-d283-7804-bd52-98043287afaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedPaymentArrangementEmployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019cb57f-d283-75b4-be9a-6350d0f508c6" xlink:to="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_019cb57f-d283-7804-bd52-98043287afaf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedPaymentArrangementExecutivesMember_019cb57f-d283-7553-920b-c243b9cb0752" xlink:href="fsbc-20260331.xsd#fsbc_ShareBasedPaymentArrangementExecutivesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019cb57f-d283-75b4-be9a-6350d0f508c6" xlink:to="loc_fsbc_ShareBasedPaymentArrangementExecutivesMember_019cb57f-d283-7553-920b-c243b9cb0752" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedPaymentArrangementDirectorsMember_019cb57f-d283-7055-a983-1c0e86e75f5e" xlink:href="fsbc-20260331.xsd#fsbc_ShareBasedPaymentArrangementDirectorsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019cb57f-d283-75b4-be9a-6350d0f508c6" xlink:to="loc_fsbc_ShareBasedPaymentArrangementDirectorsMember_019cb57f-d283-7055-a983-1c0e86e75f5e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019cb57f-d283-720d-bb32-161ba2d58b0c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7b29-95e7-9f5020eece87" xlink:to="loc_srt_RangeAxis_019cb57f-d283-720d-bb32-161ba2d58b0c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019cb57f-d283-720d-bb32-161ba2d58b0c_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019cb57f-d283-720d-bb32-161ba2d58b0c" xlink:to="loc_srt_RangeMember_019cb57f-d283-720d-bb32-161ba2d58b0c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019cb57f-d283-72fe-89d3-e426810b4711" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019cb57f-d283-720d-bb32-161ba2d58b0c" xlink:to="loc_srt_RangeMember_019cb57f-d283-72fe-89d3-e426810b4711" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019cb57f-d283-77f7-9b09-119ee7822406" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019cb57f-d283-72fe-89d3-e426810b4711" xlink:to="loc_srt_MinimumMember_019cb57f-d283-77f7-9b09-119ee7822406" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019cb57f-d283-7c38-b74c-9c37d7af0616" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019cb57f-d283-72fe-89d3-e426810b4711" xlink:to="loc_srt_MaximumMember_019cb57f-d283-7c38-b74c-9c37d7af0616" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019cb57f-d283-7ab9-971f-59b65745fdab" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7b29-95e7-9f5020eece87" xlink:to="loc_srt_TitleOfIndividualAxis_019cb57f-d283-7ab9-971f-59b65745fdab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019cb57f-d283-7ab9-971f-59b65745fdab_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019cb57f-d283-7ab9-971f-59b65745fdab" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019cb57f-d283-7ab9-971f-59b65745fdab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019cb57f-d283-7803-ae15-fe70da75afed" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019cb57f-d283-7ab9-971f-59b65745fdab" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019cb57f-d283-7803-ae15-fe70da75afed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OfficerMember_019cb57f-d283-7f56-a991-e0977cd7c464" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019cb57f-d283-7803-ae15-fe70da75afed" xlink:to="loc_srt_OfficerMember_019cb57f-d283-7f56-a991-e0977cd7c464" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ExecutiveOfficerMember_019cb57f-d283-76e6-a432-c06891a7cab0" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019cb57f-d283-7803-ae15-fe70da75afed" xlink:to="loc_srt_ExecutiveOfficerMember_019cb57f-d283-76e6-a432-c06891a7cab0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019cb57f-d283-799b-b52b-d060c530b89c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7b29-95e7-9f5020eece87" xlink:to="loc_us-gaap_PlanNameAxis_019cb57f-d283-799b-b52b-d060c530b89c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019cb57f-d283-799b-b52b-d060c530b89c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019cb57f-d283-799b-b52b-d060c530b89c" xlink:to="loc_us-gaap_PlanNameDomain_019cb57f-d283-799b-b52b-d060c530b89c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019cb57f-d283-70aa-a149-47d42cd961b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019cb57f-d283-799b-b52b-d060c530b89c" xlink:to="loc_us-gaap_PlanNameDomain_019cb57f-d283-70aa-a149-47d42cd961b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_EquityIncentivePlanMember_019cb57f-d283-7e38-b011-7ca1699a36c6" xlink:href="fsbc-20260331.xsd#fsbc_EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019cb57f-d283-70aa-a149-47d42cd961b2" xlink:to="loc_fsbc_EquityIncentivePlanMember_019cb57f-d283-7e38-b011-7ca1699a36c6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019cb57f-d283-7d6a-b928-dd5080a29e15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7b29-95e7-9f5020eece87" xlink:to="loc_us-gaap_AwardTypeAxis_019cb57f-d283-7d6a-b928-dd5080a29e15" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7d6a-b928-dd5080a29e15_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019cb57f-d283-7d6a-b928-dd5080a29e15" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7d6a-b928-dd5080a29e15_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7889-908d-dd270f6f946a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019cb57f-d283-7d6a-b928-dd5080a29e15" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7889-908d-dd270f6f946a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PerformanceBasedRestrictedStockUnitsPSUsMember_019cb57f-d283-7efa-96ed-8f3e757d91a0" xlink:href="fsbc-20260331.xsd#fsbc_PerformanceBasedRestrictedStockUnitsPSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7889-908d-dd270f6f946a" xlink:to="loc_fsbc_PerformanceBasedRestrictedStockUnitsPSUsMember_019cb57f-d283-7efa-96ed-8f3e757d91a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RestrictedStockAwardsRSAsMember_019cb57f-d283-7b9c-92c4-50f59382e5e2" xlink:href="fsbc-20260331.xsd#fsbc_RestrictedStockAwardsRSAsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7889-908d-dd270f6f946a" xlink:to="loc_fsbc_RestrictedStockAwardsRSAsMember_019cb57f-d283-7b9c-92c4-50f59382e5e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019de9ac-7dc4-7386-9d5d-72332116bafe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7889-908d-dd270f6f946a" xlink:to="loc_us-gaap_PerformanceSharesMember_019de9ac-7dc4-7386-9d5d-72332116bafe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionAxis_019de9ac-7dc4-7cfc-8616-fea28473ab28" xlink:href="fsbc-20260331.xsd#fsbc_ShareBasedCompensationPerformancePercentilePositionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7b29-95e7-9f5020eece87" xlink:to="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionAxis_019de9ac-7dc4-7cfc-8616-fea28473ab28" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionDomain_019de9ac-7dc4-7cfc-8616-fea28473ab28_default" xlink:href="fsbc-20260331.xsd#fsbc_ShareBasedCompensationPerformancePercentilePositionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionAxis_019de9ac-7dc4-7cfc-8616-fea28473ab28" xlink:to="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionDomain_019de9ac-7dc4-7cfc-8616-fea28473ab28_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionDomain_019de9ac-7dc4-7b5d-a9df-ff56b8910254" xlink:href="fsbc-20260331.xsd#fsbc_ShareBasedCompensationPerformancePercentilePositionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionAxis_019de9ac-7dc4-7cfc-8616-fea28473ab28" xlink:to="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionDomain_019de9ac-7dc4-7b5d-a9df-ff56b8910254" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_A60thPercentileMember_019de9ac-7dc4-7bce-911d-e271f43b577e" xlink:href="fsbc-20260331.xsd#fsbc_A60thPercentileMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionDomain_019de9ac-7dc4-7b5d-a9df-ff56b8910254" xlink:to="loc_fsbc_A60thPercentileMember_019de9ac-7dc4-7bce-911d-e271f43b577e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_A70thPercentileMember_019de9ad-af1e-7505-9b40-87af238325a9" xlink:href="fsbc-20260331.xsd#fsbc_A70thPercentileMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionDomain_019de9ac-7dc4-7b5d-a9df-ff56b8910254" xlink:to="loc_fsbc_A70thPercentileMember_019de9ad-af1e-7505-9b40-87af238325a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_A80thPercentileOrHigherMember_019de9ae-226f-7939-be0d-0b249f1eced3" xlink:href="fsbc-20260331.xsd#fsbc_A80thPercentileOrHigherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fsbc_ShareBasedCompensationPerformancePercentilePositionDomain_019de9ac-7dc4-7b5d-a9df-ff56b8910254" xlink:to="loc_fsbc_A80thPercentileOrHigherMember_019de9ae-226f-7939-be0d-0b249f1eced3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareCashPaid_019cb57f-d283-7ca0-9976-ab770f645856" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockDividendsPerShareCashPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_us-gaap_CommonStockDividendsPerShareCashPaid_019cb57f-d283-7ca0-9976-ab770f645856" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_019cb57f-d283-7226-a31c-5c7843b19bae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_us-gaap_DividendsCommonStock_019cb57f-d283-7226-a31c-5c7843b19bae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019cb57f-d283-7743-b4e7-b27d924e6728" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019cb57f-d283-7743-b4e7-b27d924e6728" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRightsAwardPayoutsPercentage_019cb57f-d283-7ca7-a119-e5aa9ecfcc82" xlink:href="fsbc-20260331.xsd#fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRightsAwardPayoutsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRightsAwardPayoutsPercentage_019cb57f-d283-7ca7-a119-e5aa9ecfcc82" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019cb57f-d283-710c-b9e5-3c5eaf7002db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_us-gaap_ShareBasedCompensation_019cb57f-d283-710c-b9e5-3c5eaf7002db" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeBenefitsAndShareBasedCompensation_019cb57f-d283-7a8f-9b14-89ecc4b7c5d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeBenefitsAndShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_us-gaap_EmployeeBenefitsAndShareBasedCompensation_019cb57f-d283-7a8f-9b14-89ecc4b7c5d6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UnvestedRestrictedShares_019cb57f-d283-7375-bb6c-3cc4e2a13528" xlink:href="fsbc-20260331.xsd#fsbc_UnvestedRestrictedShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_fsbc_UnvestedRestrictedShares_019cb57f-d283-7375-bb6c-3cc4e2a13528" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedWeightedAverageRemainingContractualTerms_019cb57f-d283-7247-bf96-ddd0f8d5451f" xlink:href="fsbc-20260331.xsd#fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedWeightedAverageRemainingContractualTerms_019cb57f-d283-7247-bf96-ddd0f8d5451f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetAwardGrantPercentage_019de9ac-7dc4-70a9-bbbf-684992967854" xlink:href="fsbc-20260331.xsd#fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetAwardGrantPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7e58-8bc7-a46521bcd11f" xlink:to="loc_fsbc_ShareBasedCompensationArrangementByShareBasedPaymentAwardTargetAwardGrantPercentage_019de9ac-7dc4-70a9-bbbf-684992967854" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/ShareholdersEquitySummaryofUnvestedSharesActivityDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#ShareholdersEquitySummaryofUnvestedSharesActivityDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/ShareholdersEquitySummaryofUnvestedSharesActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7192-9c12-55c0099c5e22" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7ca1-8450-6496bfae4007" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7192-9c12-55c0099c5e22" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7ca1-8450-6496bfae4007" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019cb57f-d283-79f7-918c-29cacb11d6fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019cb57f-d283-7ca1-8450-6496bfae4007" xlink:to="loc_us-gaap_AwardTypeAxis_019cb57f-d283-79f7-918c-29cacb11d6fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-79f7-918c-29cacb11d6fe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019cb57f-d283-79f7-918c-29cacb11d6fe" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-79f7-918c-29cacb11d6fe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7eb2-9ec7-672cf9f2f23a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019cb57f-d283-79f7-918c-29cacb11d6fe" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7eb2-9ec7-672cf9f2f23a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019cb57f-d283-76e1-bd03-24f9d617d93b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7eb2-9ec7-672cf9f2f23a" xlink:to="loc_us-gaap_RestrictedStockMember_019cb57f-d283-76e1-bd03-24f9d617d93b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019d0a19-78a7-7861-b21c-42cb2b23f4ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019cb57f-d283-7eb2-9ec7-672cf9f2f23a" xlink:to="loc_us-gaap_PerformanceSharesMember_019d0a19-78a7-7861-b21c-42cb2b23f4ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019cb57f-d283-7397-b311-d5e84a89d75b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7192-9c12-55c0099c5e22" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019cb57f-d283-7397-b311-d5e84a89d75b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019cb57f-d283-781e-9b75-0a3af14998c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019cb57f-d283-7397-b311-d5e84a89d75b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019cb57f-d283-781e-9b75-0a3af14998c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019cb57f-d283-7d73-8d23-75034055f0eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019cb57f-d283-7397-b311-d5e84a89d75b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019cb57f-d283-7d73-8d23-75034055f0eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019cb57f-d283-7653-a9ed-dea46d510330" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019cb57f-d283-7397-b311-d5e84a89d75b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019cb57f-d283-7653-a9ed-dea46d510330" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019cb57f-d283-7436-bf62-371fe71d57ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019cb57f-d283-7397-b311-d5e84a89d75b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019cb57f-d283-7436-bf62-371fe71d57ab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019cb57f-d283-7be7-8f60-75c8988a2fcd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019cb57f-d283-72b0-b5ea-b8f8e848dd3e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019cb57f-d283-7192-9c12-55c0099c5e22" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019cb57f-d283-72b0-b5ea-b8f8e848dd3e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019cb57f-d283-7d12-bbe8-a45c42d58b19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019cb57f-d283-72b0-b5ea-b8f8e848dd3e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019cb57f-d283-7d12-bbe8-a45c42d58b19" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019cb57f-d283-7cd0-8868-de0949102d5a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019cb57f-d283-72b0-b5ea-b8f8e848dd3e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019cb57f-d283-7cd0-8868-de0949102d5a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019cb57f-d283-72da-99db-0a0ab4ba80a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019cb57f-d283-72b0-b5ea-b8f8e848dd3e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019cb57f-d283-72da-99db-0a0ab4ba80a1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019cb57f-d283-71f9-8011-986568d1418c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019cb57f-d283-72b0-b5ea-b8f8e848dd3e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019cb57f-d283-71f9-8011-986568d1418c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019cb57f-d283-72ab-8fdc-cf20075bee8a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/CommitmentsandContingenciesUnfundedLoanCommitmentsandStandbyLettersofCreditDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#CommitmentsandContingenciesUnfundedLoanCommitmentsandStandbyLettersofCreditDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CommitmentsandContingenciesUnfundedLoanCommitmentsandStandbyLettersofCreditDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019cb57f-d283-7b73-b50e-74ce319e3224" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019cb57f-d283-7370-a3c4-2ab5b300cd7d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019cb57f-d283-7b73-b50e-74ce319e3224" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019cb57f-d283-7370-a3c4-2ab5b300cd7d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsAxis_019cb57f-d283-724e-8f92-1a16e5b383f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019cb57f-d283-7370-a3c4-2ab5b300cd7d" xlink:to="loc_us-gaap_OtherCommitmentsAxis_019cb57f-d283-724e-8f92-1a16e5b383f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-724e-8f92-1a16e5b383f3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OtherCommitmentsAxis_019cb57f-d283-724e-8f92-1a16e5b383f3" xlink:to="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-724e-8f92-1a16e5b383f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-7825-892b-e6f86467b113" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OtherCommitmentsAxis_019cb57f-d283-724e-8f92-1a16e5b383f3" xlink:to="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-7825-892b-e6f86467b113" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_CommercialLinesOfCreditMember_019d8661-db35-79d3-9104-b0e7893380e2" xlink:href="fsbc-20260331.xsd#fsbc_CommercialLinesOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-7825-892b-e6f86467b113" xlink:to="loc_fsbc_CommercialLinesOfCreditMember_019d8661-db35-79d3-9104-b0e7893380e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UndisbursedCommercialRealEstateLoansMember_019d8661-db35-75ff-8133-73239b402bcd" xlink:href="fsbc-20260331.xsd#fsbc_UndisbursedCommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-7825-892b-e6f86467b113" xlink:to="loc_fsbc_UndisbursedCommercialRealEstateLoansMember_019d8661-db35-75ff-8133-73239b402bcd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UndisbursedConstructionLoansMember_019d8661-db35-7189-ae4d-28cb2427c304" xlink:href="fsbc-20260331.xsd#fsbc_UndisbursedConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-7825-892b-e6f86467b113" xlink:to="loc_fsbc_UndisbursedConstructionLoansMember_019d8661-db35-7189-ae4d-28cb2427c304" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_AgriculturalLinesOfCreditMember_019d8661-db35-74eb-9f02-11b7310d04e9" xlink:href="fsbc-20260331.xsd#fsbc_AgriculturalLinesOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-7825-892b-e6f86467b113" xlink:to="loc_fsbc_AgriculturalLinesOfCreditMember_019d8661-db35-74eb-9f02-11b7310d04e9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UndisbursedResidentialRealEstateLoansMember_019d8661-db35-73bf-ad56-e7e935ef8ca0" xlink:href="fsbc-20260331.xsd#fsbc_UndisbursedResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-7825-892b-e6f86467b113" xlink:to="loc_fsbc_UndisbursedResidentialRealEstateLoansMember_019d8661-db35-73bf-ad56-e7e935ef8ca0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_UndisbursedAgriculturalRealEstateLoansMember_019d1f52-7c9e-7954-887c-a42c4448779c" xlink:href="fsbc-20260331.xsd#fsbc_UndisbursedAgriculturalRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-7825-892b-e6f86467b113" xlink:to="loc_fsbc_UndisbursedAgriculturalRealEstateLoansMember_019d1f52-7c9e-7954-887c-a42c4448779c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_OtherCommitmentsMember_019cb57f-d283-7da2-b97d-308ef40d3021" xlink:href="fsbc-20260331.xsd#fsbc_OtherCommitmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019cb57f-d283-7825-892b-e6f86467b113" xlink:to="loc_fsbc_OtherCommitmentsMember_019cb57f-d283-7da2-b97d-308ef40d3021" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitment_019cb57f-d283-7278-8202-0edf5d720a92" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019cb57f-d283-7b73-b50e-74ce319e3224" xlink:to="loc_us-gaap_OtherCommitment_019cb57f-d283-7278-8202-0edf5d720a92" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#CommitmentsandContingenciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsTable_019cb57f-d283-79b9-9048-26246ea54abb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_us-gaap_OtherCommitmentsTable_019cb57f-d283-79b9-9048-26246ea54abb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-73e4-b18f-060f441df622" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019cb57f-d283-79b9-9048-26246ea54abb" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-73e4-b18f-060f441df622" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-73e4-b18f-060f441df622_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-73e4-b18f-060f441df622" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-73e4-b18f-060f441df622_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-78b4-88ca-431dbb290bd1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019cb57f-d283-73e4-b18f-060f441df622" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-78b4-88ca-431dbb290bd1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnfundedLoanCommitmentMember_019cb57f-d283-79da-ba1c-849af0fa1b23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnfundedLoanCommitmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019cb57f-d283-78b4-88ca-431dbb290bd1" xlink:to="loc_us-gaap_UnfundedLoanCommitmentMember_019cb57f-d283-79da-ba1c-849af0fa1b23" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d283-77ce-9a14-73ddd3556cd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLosses_019cb57f-d283-77ce-9a14-73ddd3556cd4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_RealEstateRelatedLoansPercentage_019cb57f-d283-73fb-9b3e-c7b243851ad5" xlink:href="fsbc-20260331.xsd#fsbc_RealEstateRelatedLoansPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_fsbc_RealEstateRelatedLoansPercentage_019cb57f-d283-73fb-9b3e-c7b243851ad5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_Deposits5MillionOrMoreNumberOfDepositRelationships_019cb57f-d283-7472-9b2b-a87e6ee69cba" xlink:href="fsbc-20260331.xsd#fsbc_Deposits5MillionOrMoreNumberOfDepositRelationships"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_fsbc_Deposits5MillionOrMoreNumberOfDepositRelationships_019cb57f-d283-7472-9b2b-a87e6ee69cba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_Deposits5MillionOrMorePerDeposit_019cb57f-d283-7490-80b4-a7ac2e1aa3d3" xlink:href="fsbc-20260331.xsd#fsbc_Deposits5MillionOrMorePerDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_fsbc_Deposits5MillionOrMorePerDeposit_019cb57f-d283-7490-80b4-a7ac2e1aa3d3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_Deposits5MillionOrMoreAmount_019cb57f-d283-7754-8dda-bff2ac61b937" xlink:href="fsbc-20260331.xsd#fsbc_Deposits5MillionOrMoreAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_fsbc_Deposits5MillionOrMoreAmount_019cb57f-d283-7754-8dda-bff2ac61b937" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PercentageOfDepositsOver5MillionToDeposits_019cb57f-d283-7415-ae5b-ded612b4ec96" xlink:href="fsbc-20260331.xsd#fsbc_PercentageOfDepositsOver5MillionToDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_fsbc_PercentageOfDepositsOver5MillionToDeposits_019cb57f-d283-7415-ae5b-ded612b4ec96" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_LargestSingleDeposit_019cb57f-d283-7665-84bc-894c55435b14" xlink:href="fsbc-20260331.xsd#fsbc_LargestSingleDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_fsbc_LargestSingleDeposit_019cb57f-d283-7665-84bc-894c55435b14" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_PercentageOfLargestSingleDepositToDeposits_019cb57f-d283-7306-9c66-ced798895443" xlink:href="fsbc-20260331.xsd#fsbc_PercentageOfLargestSingleDepositToDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_fsbc_PercentageOfLargestSingleDepositToDeposits_019cb57f-d283-7306-9c66-ced798895443" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashUninsuredAmount_019cb57f-d283-7a7b-8215-d5fda92848f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashUninsuredAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_us-gaap_CashUninsuredAmount_019cb57f-d283-7a7b-8215-d5fda92848f8" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019cb57f-d283-7525-9385-0af339960180" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_us-gaap_EquityMethodInvestments_019cb57f-d283-7525-9385-0af339960180" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_fsbc_EquityMethodInvestmentRemainingCommitmentAmount_019cb57f-d283-700d-b80d-d9252183f81f" xlink:href="fsbc-20260331.xsd#fsbc_EquityMethodInvestmentRemainingCommitmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019cb57f-d283-7629-874e-53fbcaa13b2f" xlink:to="loc_fsbc_EquityMethodInvestmentRemainingCommitmentAmount_019cb57f-d283-700d-b80d-d9252183f81f" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fivestarbank.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="fsbc-20260331.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://fivestarbank.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019cb57f-d283-79de-88cc-43f4ff454554" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019cb57f-d283-7a52-98a7-8152f3809f63" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019cb57f-d283-79de-88cc-43f4ff454554" xlink:to="loc_us-gaap_SubsequentEventTable_019cb57f-d283-7a52-98a7-8152f3809f63" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019cb57f-d283-7e9e-9b0b-2e528352e23a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019cb57f-d283-7a52-98a7-8152f3809f63" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019cb57f-d283-7e9e-9b0b-2e528352e23a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019cb57f-d283-7e9e-9b0b-2e528352e23a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019cb57f-d283-7e9e-9b0b-2e528352e23a" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019cb57f-d283-7e9e-9b0b-2e528352e23a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019cb57f-d283-7418-a4a9-ecc8aebf50e4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019cb57f-d283-7e9e-9b0b-2e528352e23a" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019cb57f-d283-7418-a4a9-ecc8aebf50e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019cb57f-d283-726b-b9d0-6a9939e04f3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019cb57f-d283-7418-a4a9-ecc8aebf50e4" xlink:to="loc_us-gaap_SubsequentEventMember_019cb57f-d283-726b-b9d0-6a9939e04f3b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareDeclared_019cb57f-d283-7639-9cf9-40eb072fa192" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockDividendsPerShareDeclared"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019cb57f-d283-79de-88cc-43f4ff454554" xlink:to="loc_us-gaap_CommonStockDividendsPerShareDeclared_019cb57f-d283-7639-9cf9-40eb072fa192" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
