What is Form 6166 and when do I need a U.S. tax residency certificate?

Form 6166 is a letter from the IRS certifying that you are a U.S. resident for income tax treaty purposes. Foreign governments and financial institutions often require it before they will grant you a treaty-reduced withholding rate on dividends, interest, royalties, or pensions paid from that country. You request it by filing Form 8802, the Application for United States Residency Certification, paying a user fee, and allowing several weeks for processing, so apply well ahead of when the foreign payer needs it. This matters to immigrants who still receive income from their home country and want the lower treaty withholding rate rather than the full domestic rate. Without the certificate, the foreign payer may withhold at the higher statutory rate, forcing you to rely more heavily on the Foreign Tax Credit. See the Form 8802 instructions on irs.gov.