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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-02-23</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>2015-02-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1965</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>37685</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">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 and become issuable in three successive equal monthly installments on March 15, 2015, April 15, 2015 and May 15, 2015, provided the Reporting Person continues in the Issuer's service through each applicable vesting 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 1,965 shares subject to the reported restricted stock unit award.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Howard G. Phanstiel</signatureName>
        <signatureDate>2015-02-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
