This article is about identification numbers used in the US tax system. For taxpayer identification numbers used in other countries, see National identification number. A Taxpayer Identification Number (TIN) is an identifying number used for tax purposes in the United States.It is also known as a Tax Identification Number or Federal Taxpayer Identification Number. A TIN may be assigned by the Social Security Administration or by the Internal Revenue Service (IRS).his article is about identification numbers used in the US tax system. For taxpayer identification numbers used in other countries, see National identification number. A Taxpayer Identification Number (TIN) is an identifying number used for tax purposes in the United States.