Coordinates: 40°44′40.8″N 73°59′48.5″W / 40.744667°N 73.996806°W / 40.744667; -73.996806

The Communist Party USA (CPUSA) is a Marxist political party in the United States, established in 1919. It has a long, complex history that is closely related to the histories of similar communist parties worldwide and the U.S. labor movement.

For the first half of the 20th century, the CPUSA was the largest and most influential communist party in the United States. It played a very prominent role in the U.S. labor movement from the 1920s through the 1940s, having a major hand in founding most of the country's first industrial unions (which would later use the McCarran Internal Security Act to expel their Communist members) while also becoming known for opposing racism and fighting for integration in workplaces and communities during the height of the Jim Crow period of U.S. racial segregation. Historian Ellen Schrecker concludes that decades of recent scholarship offers a "nuanced portrayal of the party as both a Stalinist sect tied to a vicious regime and the most dynamic organization within the American Left during the 1930s and '40s". In regards to the former charge, the CPUSA, claiming proletarian internationalism (while the U.S. Government called it espionage), sponsored an elaborate intelligence network on behalf of the Soviet Union, involving over 500 members acting as agents. The most prominent example dealt with the Manhattan Project in which the network was accused in giving the blueprints of the atomic bomb to the Soviets; Julius and Ethel Rosenberg were thereafter convicted and executed as the chief architects of this plan. Subsequent examination of the trial record shows the testimony directly tying the Rosenbergs to the atomic blueprints was conflicted and possibly false.