The Union of Soviet Socialist Republics (abbreviated USSR, ; tr.: Soyuz Sovetskikh Sotsialisticheskikh Respublik, SSSR), also called the Soviet Union (Russian: Советский Союз; tr.: Sovetsky Soyuz), was a constitutionally socialist state that existed in Eurasia from 1922 to 1991.

Emerging from the Russian Empire following the Russian Revolution of 1917 and the Russian Civil War of 1918–1921, the USSR was a union of several Soviet republics, but the synecdoche Russia—after its largest and dominant constituent state—continued to be commonly used throughout the state's existence. The geographic boundaries of the USSR varied with time, but after the last major territorial annexations of the Baltic states, eastern Poland, Bessarabia, and certain other territories during World War II, from 1945 until dissolution the boundaries approximately corresponded to those of late Imperial Russia, with the notable exclusions of Poland, most of Finland, and Alaska. The Soviet Union became the primary model for future Communist states during the Cold War; the government and the political organization of the country were defined by the only political party, the Communist Party of the Soviet Union.

From 1945 until dissolution in 1991—a period known as the Cold War—the Soviet Union and the United States of America were the two world superpowers that dominated the global agenda of economic policy, foreign affairs, military operations, cultural exchange, scientific advancements including the pioneering of space exploration, and sports (including the Olympic Games and various world championships).

Initially established as a union of four Soviet Socialist Republics, the USSR grew to contain 15 constituent or "union republics" by 1956: Armenian SSR, Azerbaijan SSR, Byelorussian SSR, Estonian SSR, Georgian SSR, Kazakh SSR, Kirghiz SSR, Latvian SSR, Lithuanian SSR, Moldavian SSR, Russian SFSR, Tajik SSR, Turkmen SSR, Ukrainian SSR, and Uzbek SSR. (From annexation of the Estonian SSR on August 6, 1940 up to the reorganisation of the Karelo-Finnish SSR into the Karelian ASSR on July 16, 1956, the count of "union republics" was 16.)