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

    <schemaVersion>X0202</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2006-03-15</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000203248</issuerCik>
        <issuerName>SOUTHERN UNION CO</issuerName>
        <issuerTradingSymbol>SUG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001357322</rptOwnerCik>
            <rptOwnerName>Strehl Craig F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SOUTHERN UNION COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>5444 WESTHEIMER ROAD</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77056</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>SVP Gas Services</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Restricted Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9961</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On March 15, 2006, the Board of Directors appointed Mr. Strehl Senior Vice President, Gas Services of the Company and President of Southern Union Gas Services, Ltd. and related entities.  On March 15, 2006, Mr. Strehl owned 9,961 restricted shares of the Company's common stock.  The restricted shares vest in full on March 1, 2008, provided Mr. Strehl continues his employment with the Company through this date and the additional terms and conditions of the restricted stock award agreement between Mr. Strehl and the Company are met.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Robert M. Kerrigan, III for Craig F. Strehl</signatureName>
        <signatureDate>2006-03-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
