The National Maritime Union (NMU) was an American labor union representing merchant seamen. It was founded in May 1937 by Joseph Curran, Ferdinand Smith, and M. Hedley Stone after a split from the International Seafarer's Union, AFL. It affiliated with the Congress of Industrial Organizations [CIO] at its first convention in July 1937. It was at this same convention that approximately 30,000 seamen left the ISU to join the NMU. By the end of the year, the NMU had over 50,000 members and contracts with most American shipping concerns.

Joseph Curran was elected president of the NMU and served as such until 1981. Ferdinand Smith, a Jamaican-born man of Afro-Caribbean descent, was its first vice-president; M. Hedley Stone was its first secretary-treasurer. The leadership of the NMU had strong Communist ties. Among the notable reforms achieved by the union's Communist-dominated leadership was 'checkerboarding,' the side-by-side racial integration of sailors' sleeping quarters. Another innovation of the new union was the formation of hiring halls in each port. The hiring halls ensured a steady supply of experienced seamen for passenger and cargo ships, and reduced the corruption which plagued the hiring of able seamen. The hiring halls also worked to combat racial discrimination and promote racial harmony among maritime workers. By the end of World War II, the NMU had nearly 100,000 members.

During World War II, the alliance of Communists and non-Communists in the union was weakened. The Cold War exacerbated the ideological divide, and in 1948, the NMU's Communist leadership and its allies were defeated in union elections and expelled. Joseph Curran had distanced himself from the communist elements and in fact helped purge the NMU of any Communist-affiliated members.

The NMU merged with the Seafarers International Union of North America in 2001.