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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-06-05</periodOfReport>

    <issuer>
        <issuerCik>0001142701</issuerCik>
        <issuerName>UNITED ONLINE INC</issuerName>
        <issuerTradingSymbol>UNTD</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001069411</rptOwnerCik>
            <rptOwnerName>PHANSTIEL HOWARD G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O UNITED ONLINE, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>21255 BURBANK BOULEVARD SUITE 400</rptOwnerStreet2>
            <rptOwnerCity>WOODLAND HILLS</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>91367</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2015-06-05</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>7138</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>44823</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">This represents an award of restricted stock units. Each unit will entitle the Reporting Person to one share of the Issuer's common stock upon vesting. The units will vest in full on the date of the 2016 annual stockholders meeting
(expected to occur in June 2016), subject to the Reporting Person's continued service with the Issuer through such date.</footnote>
        <footnote id="F2">Includes 1,701 shares subject to a restricted stock unit award granted on January 2, 2015 that will be issued as those units vest.</footnote>
        <footnote id="F3">Includes 7,138 shares subject to the reported restricted stock unit award.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Howard G. Phanstiel</signatureName>
        <signatureDate>2015-06-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
