A W-9 form is an IRS tax form used to collect income tax identification information from taxpayers who are being paid $600 or more during a year by a third party who must file an information return with the Internal Revenue Service. 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.

The W-9 form 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. Financial institutions sometimes send Form W-9 to a customer to request information. However, it is not necessarily required for the customer to fill out the form if the institution already has the requested information from when the customer opened an account.

In summary, a W-9 form is a formal written request for information used to confirm a persons name, address, and taxpayer identification number (TIN) for employment or other income-related purposes. It is used to furnish tax information from one party to another and is 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.