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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-04-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000713671</issuerCik>
        <issuerName>DNB FINANCIAL CORP /PA/</issuerName>
        <issuerTradingSymbol>DNBF</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001235874</rptOwnerCik>
            <rptOwnerName>GRIESSER GERARD F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>WILLOW FINANCIAL BANCORP INC.</rptOwnerStreet1>
            <rptOwnerStreet2>170 SOUTH WARNER ROAD</rptOwnerStreet2>
            <rptOwnerCity>WAYNE</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19087</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>DNB Common Stock,$1 par value,stock option</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>6.93</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2010-04-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1800</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>6.93</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2014-04-23</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2017-04-23</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>DNB Common Stock,$1 par value</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1800</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1800</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Options to purchase common stock under Registrant's 1995 Stock Option Plan with a 7-year term. Grantee first entitled to exercise on the earlier of fourth anniversary of Grant Date,date of death,termination of service,or disability, a change in control, or attainment of the current mandatory retirement age for directors. Rights forfeited on termination of service as a director prior to the first of such events. The Grantee is subject to a two-year holding period at the time of exercise, but may sell shares to pay withholding taxes. The Grantee is also restricted from exercising the options until the market value of the outstanding common shares of the Registrant reaches 115% of the exercise price of $6.93 per share.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Gerald F Sopp for Gerard F Griesser</signatureName>
        <signatureDate>2010-05-13</signatureDate>
    </ownerSignature>
</ownershipDocument>
