A W-9 tax form is an Internal Revenue Service (IRS) form used to confirm a persons name, address, and taxpayer identification number (TIN) for employment or other income-related purposes. The form is officially called Form W-9, Request for Taxpayer Identification Number and Certification, and is used by a third party who must file an information return with the IRS. The form requests the name, address, and taxpayer identification information of a taxpayer in the form of a Social Security Number or Employer Identification Number. The form is never actually sent to the IRS; it is maintained by the individual who files the information return for verification purposes. The information on the Form W-9 and the payment made are reported on a Form 1099.

Form W-9 is most commonly used in a business–contractor arrangement. Businesses can use Form W-9 to request information from contractors they hire. When a business pays a contractor in excess of $600 during a tax year, the business is required to file Form 1099-MISC, a variant of Form 1099. To fill out Form 1099-MISC, the business may need to request information (such as address and Tax Identification Number) from the contractor, for which Form W-9 is used.

In an employer–employee arrangement, Form W-9 and Form 1099 should not be used. Instead, the corresponding Form W-4 (to provide information) and Form W-2 (to report amount paid) should be filed instead. However, an employer may still send Form W-9 to have the information on record that the payee doesnt need to be sent Form 1099.

Form W-9 is used to furnish tax information from one party to another. Its a way of formally gathering personal information, having a standard method of transmitting information, and requiring the furnisher to certify that the information they are providing is correct.