A form nonresident aliens give to U.S. payers to claim foreign status and treaty benefits.
Form W-8BEN certifies to a U.S. financial institution or payer that you are a foreign person, which sets the correct withholding on U.S.-source income such as interest or dividends and lets you claim reduced rates under a tax treaty. It is the nonresident counterpart to the W-9 that U.S. persons provide.
A nonresident investor files a W-8BEN with a U.S. broker to claim a treaty-reduced withholding rate.