IRS Form W-9 is used to give a payer, transfer agent, broker, or other middleman the Social Security or taxpayer identification number of an employee or other payee. The form is needed to certify that you are providing the right name and number and that you are not subject to backup withholding because of underreporting of interest and dividends on your tax return (unless you are a US exempt payee). You complete this form in situations such as setting up a brokerage account. The form is used for all US tax residents (both for citizens and for resident aliens with a Green Card).

Nonresidents use Form W-8BEN. US citizens living and working abroad must still complete Form W-9 and not Form W-8BEN.