The U.S. Internal Revenue Code (IRC) requires the Social Security Administration (SSA) to withhold nonresident alien tax from certain Social Security monthly benefits. If you are a nonresident alien receiving retirement, disability or survivors benefits, SSA will withhold a 30 percent flat tax from 85 percent of those benefits unless you qualify for a tax treaty benefit. This results in a withholding of 25.5 percent of your monthly benefit amount.
SSA acts as the withholding agent for the Internal Revenue Service (IRS) and applies their laws and definitions in withholding this tax. You can read more about federal income taxes and your social security benefits.
This site contains a Nonresident Alien Tax Screening Tool. By answering a few questions in this tool, you can learn whether SSA should withhold nonresident alien tax from the Social Security benefits that are payable to you.