The United States Immigration and Naturalization Service (INS) was an agency of the U.S. Department of Labor from 1933 to 1940 and the U.S. Department of Justice from 1940 to 2003.

Referred to by some as former INS and by others as legacy INS, the agency ceased to exist under that name on March 1, 2003, when most of its functions were transferred to three new entities – U.S. Citizenship and Immigration Services (USCIS), U.S. Immigration and Customs Enforcement (ICE), and U.S. Customs and Border Protection (CBP) – within the newly created Department of Homeland Security (DHS), as part of a major government reorganization following the September 11 attacks of 2001.

Prior to 1933, there were separate offices administering immigration and naturalization matters, known as the Bureau of Immigration and the Bureau of Naturalization, respectively. The INS was established on June 10, 1933, merging these previously separate areas of administration. In 1890, the federal government, rather than the individual states, regulated immigration into the United States, and the Immigration Act of 1891 established a Commissioner of Immigration in the Treasury Department. Reflecting changing governmental concerns, immigration was transferred to the purview of the United States Department of Commerce and Labor after 1903 and the Department of Labor after 1913. In 1940, with increasing concern about national security, immigration and naturalization was organized under the authority of the Department of Justice.

In 2003 the administration of immigration services, including permanent residence, naturalization, asylum, and other functions, became the responsibility of the Bureau of Citizenship and Immigration Services (BCIS), which existed under that name only for a short time before changing to its current name, U.S. Citizenship and Immigration Services (USCIS). The investigative and enforcement functions of the INS (including investigations, deportation, and intelligence) were combined with the U.S. Customs investigators to create U.S. Immigration and Customs Enforcement (ICE). The border functions of the INS, which included the Border Patrol and INS Inspectors, were combined with U.S. Customs Inspectors to create U.S. Customs and Border Protection (CBP).