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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2007-03-29</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000099047</issuerCik>
        <issuerName>NYFIX INC</issuerName>
        <issuerTradingSymbol>nyfx</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001387982</rptOwnerCik>
            <rptOwnerName>Merrill David A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>100 WALL STREET</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10005</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Global Head Client Operations</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2007-03-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>48169</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>6.228</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>48169</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">NYFIX issued these shares to Mr. Merrill in satisfaction of a provision in Mr. Merrill's January 5, 2007 offer letter that the Company, within three months of his commencement of employment, provide him a restricted stock grant with a value of $300,000 on the date of grant or pay him $300,000 in cash.  These restricted shares vest in 12 months from the date of grant.  Mr. Merrill will forfeit such grant if he is not employed by the Company on the date the award is to vest (unless he is terminated by the Company without Cause, as defined in his January 5, 2007 offer letter).</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Brian Bellardo, by Power of Attorney</signatureName>
        <signatureDate>2007-04-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2007-03-29
0
0000099047
NYFIX INC
nyfx
0001387982
Merrill David A
100 WALL STREET
NEW YORK
NY
10005
0
1
0
0
Global Head Client Operations
Common Stock
2007-03-29
4
A
0
48169
6.228
A
48169
D
NYFIX issued these shares to Mr. Merrill in satisfaction of a provision in Mr. Merrill's January 5, 2007 offer letter that the Company, within three months of his commencement of employment, provide him a restricted stock grant with a value of $300,000 on the date of grant or pay him $300,000 in cash.  These restricted shares vest in 12 months from the date of grant.  Mr. Merrill will forfeit such grant if he is not employed by the Company on the date the award is to vest (unless he is terminated by the Company without Cause, as defined in his January 5, 2007 offer letter).
Brian Bellardo, by Power of Attorney
2007-04-02