The Socialist Workers Party, or SWP, is a communist political party in the United States. It is well known on the U.S. Radical Left for having been the largest and most active promoter of Trotskyism in that country for about half of the 20th century. In the late 1980s, the SWP's supporters internationally reconstituted themselves in each country as under the name of the Communist League - renaming national sections of the USFI, or splitting from them, or being expelled. In 2003, the party sold its major headquarters building in New York City and moved to another location in Manhattan.

The SWP has continued the policies known as the "turn to industry" and a large majority of its members are industrial workers and trade union members. It places a considerable priority on participation in, and solidarity work to aid, strikes and other labor disputes. One of the SWP's main priorities is supporting Pathfinder Press, which publishes titles by SWP leaders (Cannon, Dobbs, Reed, Barnes) as well as by revolutionaries from Vladimir Lenin and Leon Trotsky to Malcolm X and Ernesto "Che" Guevara.

The Socialist Workers Party, founded in 1938, traces its origins back to the former Communist League of America. The CLA had been founded in 1928 by members of the Communist Party USA expelled for supporting Russian Communist leader Leon Trotsky against Joseph Stalin. In 1934, the Communist League of America merged with the American Workers Party led by A.J. Muste, forming the U.S Workers Party. Many members of that organization, in turn, joined the Socialist Party of America in 1936. The Socialist Party soon expelled the former Workers Party members, along with others recruited to their Trotskyist politics.

Those expelled founded the Socialist Workers Party, combining the names of the Workers Party and the Socialist Party. The new party participated in founding the Fourth International.

The SWP's best known leader was James Patrick Cannon, a former member of the Industrial Workers of the World and former head of the International Labor Defense. Another prominent leader was Max Shachtman until a split between the two men in 1940.

In the United States, the largest strike wave in U.S. history - involving over five million workers - occurred with the end of the war and the wartime pledge made by many union leaders not to strike for the duration. (This did not mean there were not many strikes during wartime - there were many wildcat strikes during this period, as well as strikes officially called by the United Mine Workers of America. There were also protests by GIs demanding rapid demobilization after the end of the war, sometimes called the going-home movement). SWP participation in this upsurge led to a brief period of rapid growth for the SWP immediately after the war.