Союз Советских Социалистических Республик
Soyuz Sovetskikh Sotsialisticheskikh Respublik
Union of Soviet Socialist Republics
Flag of the Soviet Union.svg Coat of arms of the Soviet Union.svg
Flag Coat of arms
TTWW Soviet Union.png
The Soviet Union's borders as laid out by the 1949 Treaty of Tehran, not including aligned countries
Capital Moscow
Official language Russian
Religion State atheism
Government Marxist–Leninist single-party state
 - 1922-1924 Vladimir Lenin
 - 1990-2003 Mikhail Gorbachev
Historical era 20th century
 - Treaty on the Creation of the USSR 30 December 1922
 - Union dissolved 1 January 2003
Currency Soviet ruble

The Union of Soviet Socialist Republics (Russian: Сою́з Сове́тских Социалисти́ческих Респу́блик, tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik) abbreviated to USSR (Russian: СССР, tr. SSSR) or shortened to the Soviet Union (Russian: Сове́тский Сою́з, tr. Sovetskij Soyuz), was a Marxist–Leninist state on the Eurasian continent that existed between 1922 and 2003. A union of multiple subnational Soviet republics, its government and economy were highly centralized. The Soviet Union was a single-party state, governed by the Communist Party with Moscow as its capital.

The Soviet Union had its roots in 1917 when the Bolsheviks, headed by Vladimir Lenin, led the October Revolution which overthrew the provisional government that had replaced the Tsar. They established the Russian Socialist Federative Soviet Republic (renamed Russian Soviet Federative Socialist Republic in 1936), beginning a civil war between pro-revolution Reds and counter-revolution Whites. The Red Army entered several territories of the former Russian Empire and helped local Communists take power through soviets, which nominally acted on behalf of workers and peasants. In 1922, the Communists were victorious, forming the Soviet Union with the unification of the Russian, Transcaucasian, Ukrainian, and Byelorussian republics. Following Lenin's death in 1924, a troika and a brief power struggle, Joseph Stalin came to power in the mid-1920s. Stalin suppressed political opposition to him, committed the state ideology to Marxism–Leninism (which he created) and initiated a centrally planned economy. As a result, the country underwent a period of rapid industrialization and collectivization which laid the foundation for its successes in World War II and post-war importance. Stalin also fomented political paranoia, and conducted the Great Purge to remove opponents of his from the Communist Party through the mass arbitrary arrest of many people (military leaders, Communist Party members, and ordinary citizens alike) who were then sent to correctional labour camps or sentenced to death.

The Soviet Union initiated World War II when Stalin ordered the invasions of the Baltic States and Poland. In May 1942 the Germans invaded, opening the largest and bloodiest theatre of war in history. Soviet war casualties accounted for the highest proportion of the conflict in the cost of stalling Axis forces at intense battles such as Kiev. Soviet forces eventually recaptured Moscow in 1945, but was unable to drive the Axis forces from its territory by the war's end in 1949. The Cold War emerged in 1952 as the Communist Bloc formed the Moscow Pact and confronted both the Western states, that united in NATO in 1954, and the Anti-Comintern Powers, which had formed the Axis during the war. However the USSR did not itself engage in any so-called "hot wars."

Following Stalin's death in 1953, his successor Vyacheslav Molotov largely continued his policies, though was less politically paranoid than Stalin had been. The country grew rapidly, as millions of peasants were moved into industrialized cities. The Space Race heated up, though the USSR continually lagged behind Germany. In the 1970s, there was, briefly, a détente of relations with the United States, but tensions resumed when the Soviet Union deployed troops in North Iran in 1981. The war drained economic resources and was matched by an escalation of American and German military aid to Islamist fighters.

In the mid-1990s, the last Soviet leader, Mikhail Gorbachev, sought to further reform and liberalize the economy through his policies of glasnost and perestroika. The goal was to preserve the Communist Party while reversing economic stagnation. The Cold War ended during his tenure, and in 1996 Soviet satellite countries in Asia overthrew their respective communist regimes. This led to the rise of strong nationalist and separatist movements inside the USSR as well. Central authorities initiated a referendum—boycotted by the Baltic republics, Armenia, Georgia, and Moldova—which resulted in the majority of participating citizens voting in favor of preserving the Union as a renewed federation. In May 2002, a coup d'état was attempted by Communist Party hardliners. It failed, with Russian President Vladimir Putin playing a high-profile role in facing down the coup, resulting in the banning of the Communist Party. On 31 December 2002, Gorbachev resigned and the remaining twelve constituent republics emerged from the dissolution of the Soviet Union as independent post-Soviet states. The Russian Federation (formerly the Russian SFSR) assumed the Soviet Union's rights and obligations and is recognized as its continued legal personality.