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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019ecc17-f362-73e0-91e7-bb37766c8072,g:7a8ae974-4d84-49a1-b7ba-9373c733832b-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:stba="http://www.stbank.com/20260626" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2024-01-31" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://www.stbank.com/20260626">
  <xs:import namespace="http://fasb.org/srt/2026" schemaLocation="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd"/>
  <xs:import namespace="http://fasb.org/us-gaap-ebp/2026" schemaLocation="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd"/>
  <xs:import namespace="http://fasb.org/us-gaap/2026" schemaLocation="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/dtr/type/2024-01-31" schemaLocation="https://www.xbrl.org/dtr/type/2024-01-31/types.xsd"/>
  <xs:import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/dei/2026" schemaLocation="https://xbrl.sec.gov/dei/2026/dei-2026.xsd"/>
  <xs:annotation>
    <xs:appinfo>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="stba-20260626_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="stba-20260626_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="stba-20260626_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="stba-20260626_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:type="simple"/>
      <link:roleType id="Cover" roleURI="http://www.stbank.com/role/Cover">
        <link:definition>0000001 - Document - Cover</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StatementsofNetAssetsAvailableforBenefits" roleURI="http://www.stbank.com/role/StatementsofNetAssetsAvailableforBenefits">
        <link:definition>0000002 - Statement - Statements of Net Assets Available for Benefits</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StatementsofNetAssetsAvailableforBenefits_1" roleURI="http://www.stbank.com/role/StatementsofNetAssetsAvailableforBenefits_1">
        <link:definition>0000002 - Statement - Statements of Net Assets Available for Benefits</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StatementofChangesinNetAssetsAvailableforBenefits" roleURI="http://www.stbank.com/role/StatementofChangesinNetAssetsAvailableforBenefits">
        <link:definition>0000003 - Statement - Statement of Changes in Net Assets Available for Benefits</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DescriptionofthePlan" roleURI="http://www.stbank.com/role/DescriptionofthePlan">
        <link:definition>0000004 - Disclosure - Description of the Plan</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPolicies" roleURI="http://www.stbank.com/role/SummaryofSignificantAccountingPolicies">
        <link:definition>0000005 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="TransactionswithPartiesinInterest" roleURI="http://www.stbank.com/role/TransactionswithPartiesinInterest">
        <link:definition>0000006 - Disclosure - Transactions with Parties-in-Interest</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxStatus" roleURI="http://www.stbank.com/role/IncomeTaxStatus">
        <link:definition>0000007 - Disclosure - Income Tax Status</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RisksandUncertainties" roleURI="http://www.stbank.com/role/RisksandUncertainties">
        <link:definition>0000008 - Disclosure - Risks and Uncertainties</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PlanTermination" roleURI="http://www.stbank.com/role/PlanTermination">
        <link:definition>0000009 - Disclosure - Plan Termination</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ScheduleHLine4aScheduleofDelinquentParticipantContributions" roleURI="http://www.stbank.com/role/ScheduleHLine4aScheduleofDelinquentParticipantContributions">
        <link:definition>0000010 - Disclosure - Schedule H, Line 4a - Schedule of Delinquent Participant Contributions</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesPolicies" roleURI="http://www.stbank.com/role/SummaryofSignificantAccountingPoliciesPolicies">
        <link:definition>0000011 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesTables" roleURI="http://www.stbank.com/role/SummaryofSignificantAccountingPoliciesTables">
        <link:definition>0000012 - Disclosure - Summary of Significant Accounting Policies (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ScheduleHLine4iScheduleofAssetsHeldAtEndofYearTables" roleURI="http://www.stbank.com/role/ScheduleHLine4iScheduleofAssetsHeldAtEndofYearTables">
        <link:definition>0000013 - Disclosure - Schedule H, Line 4i &#8211; Schedule of Assets (Held At End of Year) (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DescriptionofthePlanDetails" roleURI="http://www.stbank.com/role/DescriptionofthePlanDetails">
        <link:definition>0000014 - Disclosure - Description of the Plan (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesFairValueMeasurementsDetails" roleURI="http://www.stbank.com/role/SummaryofSignificantAccountingPoliciesFairValueMeasurementsDetails">
        <link:definition>0000015 - Disclosure - Summary of Significant Accounting Policies - Fair Value Measurements (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="TransactionswithPartiesinInterestDetails" roleURI="http://www.stbank.com/role/TransactionswithPartiesinInterestDetails">
        <link:definition>0000016 - Disclosure - Transactions with Parties-in-Interest (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ScheduleHLine4iScheduleofAssetsHeldAtEndofYearDetails" roleURI="http://www.stbank.com/role/ScheduleHLine4iScheduleofAssetsHeldAtEndofYearDetails">
        <link:definition>0000017 - Disclosure - Schedule H, Line 4i &#8211; Schedule of Assets (Held At End of Year) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ScheduleHLine4iScheduleofAssetsHeldAtEndofYearDetails_1" roleURI="http://www.stbank.com/role/ScheduleHLine4iScheduleofAssetsHeldAtEndofYearDetails_1">
        <link:definition>0000017 - Disclosure - Schedule H, Line 4i &#8211; Schedule of Assets (Held At End of Year) (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ScheduleHLine4aScheduleofDelinquentParticipantContributionsDetails" roleURI="http://www.stbank.com/role/ScheduleHLine4aScheduleofDelinquentParticipantContributionsDetails">
        <link:definition>0000018 - Disclosure - Schedule H, Line 4a - Schedule of Delinquent Participant Contributions (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xs:appinfo>
  </xs:annotation>
  <xs:element id="stba_EBP002Member" abstract="true" name="EBP002Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="stba_EBPAdministrativeFeesPolicyPolicyTextBlock" abstract="false" name="EBPAdministrativeFeesPolicyPolicyTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="stba_EBPEmployerContributionTypeAxis" abstract="true" name="EBPEmployerContributionTypeAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="stba_EBPEmployerContributionTypeDomain" abstract="true" name="EBPEmployerContributionTypeDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="stba_EmployerMatchingContributionsFirstOnePercentOfEligibleCompensationMember" abstract="true" name="EmployerMatchingContributionsFirstOnePercentOfEligibleCompensationMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="stba_EmployerMatchingContributionsTwoToSixPercentOfEligibleCompensationMember" abstract="true" name="EmployerMatchingContributionsTwoToSixPercentOfEligibleCompensationMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="stba_EBPFixedRateContributionMember" abstract="true" name="EBPFixedRateContributionMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="stba_EBPParticipantContributionAfterTaxAnnualContributionToEligibleCompensationPercentage" abstract="false" name="EBPParticipantContributionAfterTaxAnnualContributionToEligibleCompensationPercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="stba_EBPParticipantContributionAutomaticAnnualIncreaseDeferralRate" abstract="false" name="EBPParticipantContributionAutomaticAnnualIncreaseDeferralRate" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="stba_EBPEmployerDiscretionaryContributionAmount" abstract="false" name="EBPEmployerDiscretionaryContributionAmount" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="stba_EBPEmployerContributionVestingServicePeriod" abstract="false" name="EBPEmployerContributionVestingServicePeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="stba_EBPEmployerContributionVestingPercentage" abstract="false" name="EBPEmployerContributionVestingPercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="stba_EBPPaymentOfBenefitsTerminatedParticipantsAccountBalanceThresholdAmountForMandataryRollover" abstract="false" name="EBPPaymentOfBenefitsTerminatedParticipantsAccountBalanceThresholdAmountForMandataryRollover" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="stba_EBPPaymentOfBenefitsTerminatedParticipantsAccountBalanceThresholdAmountForMandataryLumpSumDistribution" abstract="false" name="EBPPaymentOfBenefitsTerminatedParticipantsAccountBalanceThresholdAmountForMandataryLumpSumDistribution" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="stba_EBPChangeInNetAssetAvailableForBenefitTransferToFromPlanTransfersFeesAndForfeitures" abstract="false" name="EBPChangeInNetAssetAvailableForBenefitTransferToFromPlanTransfersFeesAndForfeitures" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
</xs:schema>
