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

    <schemaVersion>X0704</schemaVersion>

    <submissionType>D</submissionType>

    <testOrLive>LIVE</testOrLive>

    <primaryIssuer>
        <cik>0001164994</cik>
        <entityName>RIB X PHARMACEUTICALS INC</entityName>
        <issuerAddress>
            <street1>300 GEORGE STREET</street1>
            <street2>SUITE 301</street2>
            <city>NEW HAVEN</city>
            <stateOrCountry>CT</stateOrCountry>
            <stateOrCountryDescription>CONNECTICUT</stateOrCountryDescription>
            <zipCode>06511</zipCode>
        </issuerAddress>
        <issuerPhoneNumber>203-624-5606</issuerPhoneNumber>
        <jurisdictionOfInc>DELAWARE</jurisdictionOfInc>
        <issuerPreviousNameList>
            <value>None</value>
        </issuerPreviousNameList>
        <edgarPreviousNameList>
            <value>None</value>
        </edgarPreviousNameList>
        <entityType>Corporation</entityType>
        <yearOfInc>
            <overFiveYears>true</overFiveYears>
        </yearOfInc>
    </primaryIssuer>

    <relatedPersonsList>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Mark</firstName>
                <lastName>Leuchtenberger</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o Rib-X Pharmaceuticals, Inc.</street1>
                <street2>300 George Street, Suite 301</street2>
                <city>New Haven</city>
                <stateOrCountry>CT</stateOrCountry>
                <stateOrCountryDescription>CONNECTICUT</stateOrCountryDescription>
                <zipCode>06511</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Robert</firstName>
                <lastName>Conerly</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o Rib-X Pharmaceuticals, Inc.</street1>
                <street2>300 George Street, Suite 301</street2>
                <city>New Haven</city>
                <stateOrCountry>CT</stateOrCountry>
                <stateOrCountryDescription>CONNECTICUT</stateOrCountryDescription>
                <zipCode>06511</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Jonathan</firstName>
                <lastName>Leff</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>WP VIII Finance, L.P. , c/o Warburg Pinc</street1>
                <street2>450 Lexington Avenue</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10017</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Cecilia</firstName>
                <lastName>Gonzalo</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>WP VIII Finance, L.P. , c/o Warburg Pinc</street1>
                <street2>450 Lexington Avenue</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10017</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Harry</firstName>
                <middleName>H.</middleName>
                <lastName>Penner, Jr.</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o Rib-X Pharmaceuticals, Inc.</street1>
                <street2>300 George Street, Suite 301</street2>
                <city>New Haven</city>
                <stateOrCountry>CT</stateOrCountry>
                <stateOrCountryDescription>CONNECTICUT</stateOrCountryDescription>
                <zipCode>06511</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Jonathan</firstName>
                <lastName>Fleming</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o Oxford Biosciences Partners IV, L.P.</street1>
                <street2>222 Berkeley Street, Suite 1650</street2>
                <city>Boston</city>
                <stateOrCountry>MA</stateOrCountry>
                <stateOrCountryDescription>MASSACHUSETTS</stateOrCountryDescription>
                <zipCode>02116</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Raymond</firstName>
                <lastName>Whitaker</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o EuclidSR Partners, L.P.</street1>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10111</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>C. Boyd</firstName>
                <lastName>Clarke</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>265 Cheswold Lane</street1>
                <city>Haverford</city>
                <stateOrCountry>PA</stateOrCountry>
                <stateOrCountryDescription>PENNSYLVANIA</stateOrCountryDescription>
                <zipCode>19041</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Peter</firstName>
                <lastName>Greenleaf</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o Rib-X Pharmaceuticals, Inc.</street1>
                <street2>300 George Street, Suite 301</street2>
                <city>New Haven</city>
                <stateOrCountry>CT</stateOrCountry>
                <stateOrCountryDescription>CONNECTICUT</stateOrCountryDescription>
                <zipCode>06511</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
    </relatedPersonsList>

    <offeringData>
        <industryGroup>
            <industryGroupType>Pharmaceuticals</industryGroupType>
        </industryGroup>
        <issuerSize>
            <revenueRange>Decline to Disclose</revenueRange>
        </issuerSize>
        <federalExemptionsExclusions>
            <item>06</item>
        </federalExemptionsExclusions>
        <typeOfFiling>
            <newOrAmendment>
                <isAmendment>false</isAmendment>
            </newOrAmendment>
            <dateOfFirstSale>
                <value>2010-05-28</value>
            </dateOfFirstSale>
        </typeOfFiling>
        <durationOfOffering>
            <moreThanOneYear>false</moreThanOneYear>
        </durationOfOffering>
        <typesOfSecuritiesOffered>
            <isDebtType>true</isDebtType>
            <isOptionToAcquireType>true</isOptionToAcquireType>
        </typesOfSecuritiesOffered>
        <businessCombinationTransaction>
            <isBusinessCombinationTransaction>false</isBusinessCombinationTransaction>
            <clarificationOfResponse></clarificationOfResponse>
        </businessCombinationTransaction>
        <minimumInvestmentAccepted>0</minimumInvestmentAccepted>
        <salesCompensationList></salesCompensationList>
        <offeringSalesAmounts>
            <totalOfferingAmount>15000000</totalOfferingAmount>
            <totalAmountSold>5500000</totalAmountSold>
            <totalRemaining>9500000</totalRemaining>
            <clarificationOfResponse></clarificationOfResponse>
        </offeringSalesAmounts>
        <investors>
            <hasNonAccreditedInvestors>false</hasNonAccreditedInvestors>
            <totalNumberAlreadyInvested>16</totalNumberAlreadyInvested>
        </investors>
        <salesCommissionsFindersFees>
            <salesCommissions>
                <dollarAmount>0</dollarAmount>
            </salesCommissions>
            <findersFees>
                <dollarAmount>0</dollarAmount>
            </findersFees>
            <clarificationOfResponse></clarificationOfResponse>
        </salesCommissionsFindersFees>
        <useOfProceeds>
            <grossProceedsUsed>
                <dollarAmount>0</dollarAmount>
                <isEstimate>true</isEstimate>
            </grossProceedsUsed>
            <clarificationOfResponse></clarificationOfResponse>
        </useOfProceeds>
        <signatureBlock>
            <authorizedRepresentative>false</authorizedRepresentative>
            <signature>
                <issuerName>RIB X PHARMACEUTICALS INC</issuerName>
                <signatureName>Robert Conerly</signatureName>
                <nameOfSigner>Robert Conerly</nameOfSigner>
                <signatureTitle>Chief Financial Officer</signatureTitle>
                <signatureDate>2010-06-10</signatureDate>
            </signature>
        </signatureBlock>
    </offeringData>
</edgarSubmission>
