A Taxpayer Identification Number (TIN) is a unique set of numbers used for tax purposes in the United States and in other countries under the Common Reporting Standard. 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). Any government-provided number that can be used in the US as a unique identifier when interacting with the IRS is a TIN, though none of them are referred to exclusively as a Taxpayer Identification Number. Some examples of TINs include:

• Social Security number (SSN)
• Individual Taxpayer Identification Number (ITIN)
• Employer Identification Number (EIN)
• Adoption Taxpayer Identification Number (ATIN)
• Preparer Tax Identification Number (PTIN)

A TIN is mandatory for anyone filing annual tax returns with the IRS, which the agency uses to track taxpayers. Filers must include the number on tax-related documents and when claiming benefits or services from the government. TINs are also required for other purposes, such as seeking employment or obtaining credit with banks.