<?xml version="1.0" encoding="UTF-8"?><edgarSubmission xmlns="http://www.sec.gov/edgar/nport" xmlns:com="http://www.sec.gov/edgar/common" xmlns:ncom="http://www.sec.gov/edgar/nportcommon" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sec.gov/edgar/nport eis_NPORT_Filer.xsd">
  <headerData>
    <submissionType>NPORT-P</submissionType>
    <isConfidential>false</isConfidential>
    <filerInfo>

      <filer>
        <issuerCredentials>
          <cik>0000783740</cik>
          <ccc>XXXXXXXX</ccc>
        </issuerCredentials>
      </filer>


      <seriesClassInfo>
        <seriesId>S000002515</seriesId>
        <classId>C000006931</classId>
        <classId>C000195683</classId>
        <classId>C000006930</classId>
        <classId>C000006933</classId>
        <classId>C000006936</classId>
        <classId>C000006935</classId>
        <classId>C000006929</classId>
        <classId>C000006924</classId>
        <classId>C000006925</classId>
      </seriesClassInfo>


    </filerInfo>
  </headerData>
  <formData>
    <genInfo>
      <regName>MFS SERIES TRUST X</regName>
      <regFileNumber>811-04492</regFileNumber>
      <regCik>0000783740</regCik>
      <regLei>5493001MUVNRNUPJY164</regLei>
      <regStreet1>111 HUNTINGTON AVENUE</regStreet1>
      <regCity>BOSTON</regCity>
      <regStateConditional regCountry="US" regState="US-MA"/>
      <regZipOrPostalCode>02199</regZipOrPostalCode>
      <regPhone>617-954-5000</regPhone>
      <seriesName>MFS International Diversification Fund</seriesName>
      <seriesId>S000002515</seriesId>
      <seriesLei>5FYSVBMHKU2K1O39U714</seriesLei>
      <repPdEnd>2025-05-31</repPdEnd>
      <repPdDate>2024-11-30</repPdDate>
      <isFinalFiling>N</isFinalFiling>
    </genInfo>
    <fundInfo>
      <totAssets>38081867368.53</totAssets>
      <totLiabs>10065099.42</totLiabs>
      <netAssets>38071802269.11</netAssets>
      <assetsAttrMiscSec>0.00000000</assetsAttrMiscSec>
      <assetsInvested>0.00000000</assetsInvested>
      <amtPayOneYrBanksBorr>0.00000000</amtPayOneYrBanksBorr>
      <amtPayOneYrCtrldComp>0.00000000</amtPayOneYrCtrldComp>
      <amtPayOneYrOthAffil>0.00000000</amtPayOneYrOthAffil>
      <amtPayOneYrOther>0.00000000</amtPayOneYrOther>
      <amtPayAftOneYrBanksBorr>0.00000000</amtPayAftOneYrBanksBorr>
      <amtPayAftOneYrCtrldComp>0.00000000</amtPayAftOneYrCtrldComp>
      <amtPayAftOneYrOthAffil>0.00000000</amtPayAftOneYrOthAffil>
      <amtPayAftOneYrOther>0.00000000</amtPayAftOneYrOther>
      <delayDeliv>0.00000000</delayDeliv>
      <standByCommit>0.00000000</standByCommit>
      <liquidPref>0.00000000</liquidPref>
      <cshNotRptdInCorD>0.00000000</cshNotRptdInCorD>
      <isNonCashCollateral>N</isNonCashCollateral>
      <returnInfo>
        <monthlyTotReturns>
          <monthlyTotReturn classId="C000006931" rtn1="2.36000000" rtn2="-4.73000000" rtn3="-0.70000000"/>
          <monthlyTotReturn classId="C000195683" rtn1="2.40000000" rtn2="-4.76000000" rtn3="-0.66000000"/>
          <monthlyTotReturn classId="C000006930" rtn1="2.27000000" rtn2="-4.84000000" rtn3="-0.73000000"/>
          <monthlyTotReturn classId="C000006933" rtn1="2.31000000" rtn2="-4.85000000" rtn3="-0.75000000"/>
          <monthlyTotReturn classId="C000006936" rtn1="2.38000000" rtn2="-4.80000000" rtn3="-0.67000000"/>
          <monthlyTotReturn classId="C000006935" rtn1="2.32000000" rtn2="-4.81000000" rtn3="-0.72000000"/>
          <monthlyTotReturn classId="C000006929" rtn1="2.31000000" rtn2="-4.83000000" rtn3="-0.76000000"/>
          <monthlyTotReturn classId="C000006924" rtn1="2.35000000" rtn2="-4.80000000" rtn3="-0.71000000"/>
          <monthlyTotReturn classId="C000006925" rtn1="2.37000000" rtn2="-4.75000000" rtn3="-0.70000000"/>
        </monthlyTotReturns>
        <othMon1 netRealizedGain="12790000.24000000" netUnrealizedAppr="929007378.72000000"/>
        <othMon2 netRealizedGain="10621365.29000000" netUnrealizedAppr="-1929786575.35000000"/>
        <othMon3 netRealizedGain="-5077451.49000000" netUnrealizedAppr="-253174243.25000000"/>
      </returnInfo>
      <mon1Flow redemption="791241334.75000000" reinvestment="0.00000000" sales="463086156.64000000"/>
      <mon2Flow redemption="725766340.56000000" reinvestment="0.00000000" sales="536192437.57000000"/>
      <mon3Flow redemption="652809584.08000000" reinvestment="0.00000000" sales="561794665.18000000"/>



    </fundInfo>
    <invstOrSecs>
      <invstOrSec>
        <name>MFS Research International Fund</name>
        <lei>UH91KW1VFNZQF5FY7237</lei>
        <title>MFS Research International Fund</title>
        <cusip>55273H361</cusip>
        <identifiers>
          <isin value="US55273H3619"/>
        </identifiers>
        <balance>446688441.58000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>10483777723.79000000</valUSD>
        <pctVal>27.53685693596</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Emerging Markets Equity Fund</name>
        <lei>GLINMRWNKCBOHYKQ2131</lei>
        <title>MFS Emerging Markets Equity Fund</title>
        <cusip>552746331</cusip>
        <identifiers>
          <isin value="US5527463310"/>
        </identifiers>
        <balance>184049157.83000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>6544788052.43000000</valUSD>
        <pctVal>17.19064415750</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS International New Discovery Fund</name>
        <lei>RC53GXLQZ5LVIZBRHR66</lei>
        <title>MFS International New Discovery Fund</title>
        <cusip>552981383</cusip>
        <identifiers>
          <isin value="US5529813835"/>
        </identifiers>
        <balance>114769323.04000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>3816079990.91000000</valUSD>
        <pctVal>10.02337626134</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS International Intrinsic Value Fund</name>
        <lei>7540QFSKYNTDFH7HAK24</lei>
        <title>MFS International Intrinsic Value Fund</title>
        <cusip>552746349</cusip>
        <identifiers>
          <isin value="US5527463492"/>
        </identifiers>
        <balance>137914854.70000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>5715191578.89000000</valUSD>
        <pctVal>15.01161289526</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Institutional Money Market Portfolio</name>
        <lei>5L2W638H85N8T1136O07</lei>
        <title>MFS Institutional Money Market Portfolio</title>
        <cusip>55291X109</cusip>
        <identifiers>
          <isin value="US55291X1090"/>
        </identifiers>
        <balance>64974215.69000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>64980713.11000000</valUSD>
        <pctVal>0.170679372231</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>STIV</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS International Large Cap Value Fund</name>
        <lei>549300YEW2NKJCMQAR75</lei>
        <title>MFS International Large Cap Value Fund</title>
        <cusip>552743445</cusip>
        <identifiers>
          <isin value="US5527434451"/>
        </identifiers>
        <balance>399147938.96000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>5727772924.12000000</valUSD>
        <pctVal>15.04465925630</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS International Growth Fund</name>
        <lei>PTNAY8DC4VJYOKVJ5B83</lei>
        <title>MFS International Growth Fund</title>
        <cusip>552746356</cusip>
        <identifiers>
          <isin value="US5527463567"/>
        </identifiers>
        <balance>127169329.10000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>5703544410.09000000</valUSD>
        <pctVal>14.98102025686</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
    </invstOrSecs>
    <explntrNotes>
      <explntrNote note="Monthly returns presented in Item B.5(a) have been calculated without deducting any applicable sales loads or redemption fees." noteItem="B.5.a"/>
    </explntrNotes>
    <signature>
      <ncom:dateSigned>2024-12-18</ncom:dateSigned>
      <ncom:nameOfApplicant>MFS SERIES TRUST X</ncom:nameOfApplicant>
      <ncom:signature>James Byrne</ncom:signature>
      <ncom:signerName>James Byrne</ncom:signerName>
      <ncom:title>Assistant Treasurer</ncom:title>
    </signature>
  </formData>
</edgarSubmission>
