The Social Security Act of 1935 is a law enacted by the 74th United States Congress and signed into law by US President Franklin D. Roosevelt. The law created the Social Security program as well as insurance against unemployment. The law was part of Roosevelt's New Deal domestic program.

By the 1930s, the United States was the only modern industrial country without any national system of social security. In the midst of the Great Depression, the physician Francis Townsend galvanized support behind a proposal to issue direct payments to the elderly. Responding to that movement, Roosevelt organized a committee led by Secretary of Labor Frances Perkins to develop a major social welfare program proposal. Roosevelt presented the plan in early 1935 and signed the Social Security Act into law on August 14, 1935. The act was upheld by the Supreme Court in two major cases decided in 1937.

The law established the Social Security program. The old-age program is funded by payroll taxes, and over the ensuing decades, it contributed to a dramatic decline in poverty among the elderly, and spending on Social Security became a major part of the federal budget. The Social Security Act also established an unemployment insurance program administered by the states and the Aid to Dependent Children program, which provided aid to families headed by single mothers. The law was later amended by acts such as the Social Security Amendments of 1965, which established two major healthcare programs: Medicare and Medicaid.