<?xml version="1.0"?>

<ownershipDocument>

    <documentType>4</documentType>

    <periodOfReport>2003-05-07</periodOfReport>


    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000751652</issuerCik>
        <issuerName>QUESTAR CORP</issuerName>
        <issuerTradingSymbol>STR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001179019</rptOwnerCik>
            <rptOwnerName>PARKS S E</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Sr.VP, Treasurer, and CFO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeSecurity>
        <securityTitle>
            <value>Phantom Stock Units</value>
        </securityTitle>
        <conversionOrExercisePrice>
            <value>31.03</value>
        </conversionOrExercisePrice>
        <transactionDate>
            <value>2003-05-07</value>
        </transactionDate>
        <transactionCoding>
            <transactionFormType>4</transactionFormType>
            <transactionCode>A</transactionCode>
            <equitySwapInvolved>0</equitySwapInvolved>
        </transactionCoding>
        <transactionTimeliness>
            <value></value>
        </transactionTimeliness>
        <transactionAmounts>
            <transactionShares>
                <value>6.961</value>
            </transactionShares>
            <transactionValue>
                <value>31.03</value>
            </transactionValue>
            <transactionAcquiredDisposedCode>
                <value>A</value>
            </transactionAcquiredDisposedCode>
        </transactionAmounts>
        <exerciseDate>
            <value>1988-08-08</value>
            <footnoteId id="F1"/>
        </exerciseDate>
        <expirationDate>
            <value>1988-08-08</value>
            <footnoteId id="F2"/>
        </expirationDate>
        <underlyingSecurity>
            <underlyingSecurityTitle>
                <value>Phantom Stock Units</value>
            </underlyingSecurityTitle>
            <underlyingSecurityShares>
                <value>6.961</value>
            </underlyingSecurityShares>
        </underlyingSecurity>
        <postTransactionAmounts>
            <sharesOwnedFollowingTransaction>
                <value>2894.83</value>
                <footnoteId id="F3"/>
            </sharesOwnedFollowingTransaction>
        </postTransactionAmounts>
        <ownershipNature>
            <directOrIndirectOwnership>
                <value>D</value>
            </directOrIndirectOwnership>
        </ownershipNature>
    </derivativeSecurity>

    <footnotes>
        <footnote id="F1">Phantom Stock Units will be converted to cash beginning at retirement; retirement date is unknown.</footnote>
        <footnote id="F2">Phantom Stock Units will be converted to cash beginning at retirement; retirement date is unknown.</footnote>
        <footnote id="F3">Total includes 1,464.3866 phantom stock units in an excess benefit plan sponsored by Questar in addition to units held through account in deferred compensation plan. This total does not include 76,949 shares held directly; 21,163.2085 equivalent shares held indirectly through Questar's Employee Investment Plan as of March 14, 2003, or 152,449 vested stock options.  (Detailed information concerning my options has been previously disclosed.)</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Connie C. Holbrook, Attorney in Fact</signatureName>
        <signatureDate>2003-05-08</signatureDate>
    </ownerSignature>

</ownershipDocument>

