<?xml version="1.0"?>
<edgarSubmission>

    <schemaVersion>X0708</schemaVersion>

    <submissionType>D/A</submissionType>

    <testOrLive>LIVE</testOrLive>

    <primaryIssuer>
        <cik>0001892401</cik>
        <entityName>CEP III - UNION PARK 21, LLC</entityName>
        <issuerAddress>
            <street1>2829 RUCKER AVENUE</street1>
            <city>EVERETT</city>
            <stateOrCountry>WA</stateOrCountry>
            <stateOrCountryDescription>WASHINGTON</stateOrCountryDescription>
            <zipCode>98201</zipCode>
        </issuerAddress>
        <issuerPhoneNumber>425-339-3638</issuerPhoneNumber>
        <jurisdictionOfInc>WASHINGTON</jurisdictionOfInc>
        <issuerPreviousNameList>
            <value>None</value>
        </issuerPreviousNameList>
        <edgarPreviousNameList>
            <value>None</value>
        </edgarPreviousNameList>
        <entityType>Limited Liability Company</entityType>
        <yearOfInc>
            <withinFiveYears>true</withinFiveYears>
            <value>2021</value>
        </yearOfInc>
    </primaryIssuer>

    <relatedPersonsList>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>THOMAS</firstName>
                <middleName>P</middleName>
                <lastName>HOBAN, JR.</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>2829 RUCKER AVENUE</street1>
                <city>EVERETT</city>
                <stateOrCountry>WA</stateOrCountry>
                <stateOrCountryDescription>WASHINGTON</stateOrCountryDescription>
                <zipCode>98201</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>SHAWN</firstName>
                <middleName>A</middleName>
                <lastName>HOBAN</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>2829 RUCKER AVENUE</street1>
                <city>EVERETT</city>
                <stateOrCountry>WA</stateOrCountry>
                <stateOrCountryDescription>WASHINGTON</stateOrCountryDescription>
                <zipCode>98201</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>MICHAEL</firstName>
                <lastName>HARMON</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>2829 RUCKER AVENUE</street1>
                <city>EVERETT</city>
                <stateOrCountry>WA</stateOrCountry>
                <stateOrCountryDescription>WASHINGTON</stateOrCountryDescription>
                <zipCode>98201</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>JOSH</firstName>
                <lastName>JANSEN</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>2829 RUCKER AVENUE</street1>
                <city>EVERETT</city>
                <stateOrCountry>WA</stateOrCountry>
                <stateOrCountryDescription>WASHINGTON</stateOrCountryDescription>
                <zipCode>98201</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
    </relatedPersonsList>

    <offeringData>
        <industryGroup>
            <industryGroupType>Other Real Estate</industryGroupType>
        </industryGroup>
        <issuerSize>
            <revenueRange>Decline to Disclose</revenueRange>
        </issuerSize>
        <federalExemptionsExclusions>
            <item>06b</item>
        </federalExemptionsExclusions>
        <typeOfFiling>
            <newOrAmendment>
                <isAmendment>true</isAmendment>
                <previousAccessionNumber>0001892401-21-000001</previousAccessionNumber>
            </newOrAmendment>
            <dateOfFirstSale>
                <value>2021-11-04</value>
            </dateOfFirstSale>
        </typeOfFiling>
        <durationOfOffering>
            <moreThanOneYear>false</moreThanOneYear>
        </durationOfOffering>
        <typesOfSecuritiesOffered>
            <isEquityType>true</isEquityType>
            <isOtherType>true</isOtherType>
            <descriptionOfOtherType>CLASS A UNITS</descriptionOfOtherType>
        </typesOfSecuritiesOffered>
        <businessCombinationTransaction>
            <isBusinessCombinationTransaction>false</isBusinessCombinationTransaction>
            <clarificationOfResponse></clarificationOfResponse>
        </businessCombinationTransaction>
        <minimumInvestmentAccepted>25000</minimumInvestmentAccepted>
        <salesCompensationList></salesCompensationList>
        <offeringSalesAmounts>
            <totalOfferingAmount>8900000</totalOfferingAmount>
            <totalAmountSold>2075000</totalAmountSold>
            <totalRemaining>6825000</totalRemaining>
            <clarificationOfResponse></clarificationOfResponse>
        </offeringSalesAmounts>
        <investors>
            <hasNonAccreditedInvestors>false</hasNonAccreditedInvestors>
            <totalNumberAlreadyInvested>11</totalNumberAlreadyInvested>
        </investors>
        <salesCommissionsFindersFees>
            <salesCommissions>
                <dollarAmount>0</dollarAmount>
            </salesCommissions>
            <findersFees>
                <dollarAmount>0</dollarAmount>
            </findersFees>
            <clarificationOfResponse></clarificationOfResponse>
        </salesCommissionsFindersFees>
        <useOfProceeds>
            <grossProceedsUsed>
                <dollarAmount>690000</dollarAmount>
                <isEstimate>true</isEstimate>
            </grossProceedsUsed>
            <clarificationOfResponse>Acquisition fee and organizational fees</clarificationOfResponse>
        </useOfProceeds>
        <signatureBlock>
            <authorizedRepresentative>false</authorizedRepresentative>
            <signature>
                <issuerName>CEP III - UNION PARK 21, LLC</issuerName>
                <signatureName>Michael Harmon</signatureName>
                <nameOfSigner>Michael Harmon, CEO Coast Equity Partners III, LLC, as Mgr.</nameOfSigner>
                <signatureTitle>CEP III - UP Manager, LLC its Manager</signatureTitle>
                <signatureDate>2021-11-08</signatureDate>
            </signature>
        </signatureBlock>
    </offeringData>
</edgarSubmission>
