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

    <schemaVersion>X0201</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2004-03-16</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001268671</issuerCik>
        <issuerName>TNS INC</issuerName>
        <issuerTradingSymbol>TNS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001262806</rptOwnerCik>
            <rptOwnerName>GTCR CO INVEST LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O GTCR GOLDER RAUNER LLC</rptOwnerStreet1>
            <rptOwnerStreet2>SEARS TOWER #6100</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606-6402</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001105021</rptOwnerCik>
            <rptOwnerName>GTCR GOLDER RAUNER LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>6100 SEARS TOWER</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606-6402</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock, par value $0.001 per share</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>86644</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By TNS Holdings, L.L.C.</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Class A Redeemable Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>82090</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By TNS Holdings, L.L.C.</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">GTCR Co-Invest, L.P. (&quot;Co-Invest&quot;) owns units of TNS Holdings, L.L.C., which is the direct beneficial owner of the shares reported in Tables I and II.  GTCR Golder Rauner, L.L.C. (&quot;GTCR&quot;) is the general partner of Co-Invest.  As such, GTCR may be deemed to be the beneficial owner of the shares reported in Tables I and II.  GTCR expressly disclaims beneficial ownership of the shares reported in Tables I and II.  The filing of this form shall not be deemed an admission that GTCR is, for Section 16 purposes or otherwise, the beneficial owner of such shares.</footnote>
        <footnote id="F2">Each share of Class A Redeemable Convertible Preferred Stock is convertible into the number of shares of Common Stock obtained by dividing (1) the sum of (a) $1,000 and (b) any accrued and unpaid dividends through March 15, 2004 by (2) the per share initial public offering price of the Common Stock, which is $18.00.</footnote>
        <footnote id="F3">The Class A Redeemable Convertible Preferred Stock is convertible upon the closing of the initial public offering of Common Stock.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Thomas N. Blanchard under Powers of Attorney attached hereto 03/16/04</signatureName>
        <signatureDate>2004-03-16</signatureDate>
    </ownerSignature>
</ownershipDocument>

