In the United States, a Social Security number (SSN) is a nine-digit number issued to U.S. citizens, permanent residents, and temporary (working) residents under section 205(c)(2) of the Social Security Act, codified as 42 U.S.C. § 405(c)(2). The number is issued to an individual by the Social Security Administration, an independent agency of the United States government. Although the original purpose for the number was for the Social Security Administration to track individuals, the Social Security number has become a de facto national identification number for taxation and other purposes.

In the United States, Social Security is the commonly used term for the federal Old-Age, Survivors, and Disability Insurance (OASDI) program and is administered by the Social Security Administration (SSA). The Social Security Act was passed in 1935, and the existing version of the Act, as amended, encompasses several social welfare and social insurance programs.