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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-03-07</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001283815</rptOwnerCik>
            <rptOwnerName>ENDOWMENT CAPITAL GROUP LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1105 NORTH MARKET STREET</rptOwnerStreet1>
            <rptOwnerStreet2>15TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>WILMINGTON</rptOwnerCity>
            <rptOwnerState>DE</rptOwnerState>
            <rptOwnerZipCode>19801</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001286800</rptOwnerCik>
            <rptOwnerName>LONG DRIVE L P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1105 NORTH MARKET STREET</rptOwnerStreet1>
            <rptOwnerStreet2>15TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>WILMINGTON</rptOwnerCity>
            <rptOwnerState>DE</rptOwnerState>
            <rptOwnerZipCode>19801</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001286807</rptOwnerCik>
            <rptOwnerName>ENDOWMENT CAPITAL L P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1105 NORTH MARKET STREET</rptOwnerStreet1>
            <rptOwnerStreet2>15TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>WILMINGTON</rptOwnerCity>
            <rptOwnerState>DE</rptOwnerState>
            <rptOwnerZipCode>19801</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001080175</rptOwnerCik>
            <rptOwnerName>TIMON PHILIP C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1105 NORTH MARKET STREET</rptOwnerStreet1>
            <rptOwnerStreet2>15TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>WILMINGTON</rptOwnerCity>
            <rptOwnerState>DE</rptOwnerState>
            <rptOwnerZipCode>19801</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001297571</rptOwnerCik>
            <rptOwnerName>Endowment Capital Group, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1105 NORTH MARKET STREET</rptOwnerStreet1>
            <rptOwnerStreet2>15TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>WILMINGTON</rptOwnerCity>
            <rptOwnerState>DE</rptOwnerState>
            <rptOwnerZipCode>19801</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2006-03-07</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>50000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>16.19</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2796500</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2006-03-08</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value>E</value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>15.878</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2801500</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Endowment Capital, L.P. (1,797,675 shares) and Long Drive, L.P. (1,003,825 shares) (collectively, the &quot;Funds&quot;) directly own the 2,801,500 shares of Common Stock (the &quot;Shares&quot;). Endowment Capital Group, LLC is the sole general partner and Endowment Capital Group, L.P. is the sole investment manager of each of the Funds. Philip Timon is the sole managing member of Endowment Capital Group, LLC and Endowment Capital Management, LLC, which is the general partner of Endowment Capital Group, L.P.</footnote>
        <footnote id="F2">By virtue of its position as the sole general partner of the Funds, Endowment Capital Group, LLC may be deemed to be the indirect beneficial owner of the Shares. Nothing in this Form 4 shall be deemed an admission that Endowment Capital Group, LLC is, for purposes of Section 16 of the Securities Act or otherwise, the beneficial owner of any securities reported on this Form 4 in which it does not have a pecuniary interest as calculated pursuant to the rules and regulations promulgated under Section 16 of the Securities Act.</footnote>
        <footnote id="F3">By virtue of its position as the investment manager of the Funds, Endowment Capital Group, L.P. may be deemed to be the indirect beneficial owner of the Shares. Nothing in this Form 4 shall be deemed an admission that Endowment Capital Group, L.P. is, for purposes of Section 16 of the Securities Act or otherwise, the beneficial owner of any securities reported on this Form 4 in which it does not have a pecuniary interest as calculated pursuant to the rules and regulations promulgated under Section 16 of the Securities Act.</footnote>
        <footnote id="F4">By virtue of his position as the managing member of both Endowment Capital Group, LLC and Endowment Capital Management, LLC, which is the general partner of Endowment Capital Group, L.P., Philip Timon may be deemed to be the indirect beneficial owner of the Shares. Nothing in this Form 4 shall be deemed an admission that Philip Timon is, for purposes of Section 16 of the Securities Act or otherwise, the beneficial owner of any securities reported on this Form 4 in which he does not have a pecuniary interest as calculated pursuant to the rules and regulations promulgated under Section 16 of the Securities Act.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>See attached Exhibit 99</signatureName>
        <signatureDate>2006-03-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
