The Byelorussian Soviet Socialist Republic (BSSR, or Byelorussian SSR; Belarusian: Беларуская Савецкая Сацыялiстычная Рэспубліка, translit. Bielaruskaja Savieckaja Sacyjalistyčnaja Respublika; Russian: Белорусская Советская Социалистическая Республика, translit. Belorusskaya Sovetskaya Sotsialisticheskaya Respublika or Белорусская ССР, lit. 'Belorusskaya SSR'), also commonly referred to in English as Byelorussia, was a federal unit of the Soviet Union (USSR). It existed between 1920 and 1922, and from 1922 to 1991 as one of fifteen constituent republics of the USSR, with its own legislation from 1990 to 1991. The republic was ruled by the Communist Party of Byelorussia and was also referred to as Soviet Byelorussia by a number of historians.
The Socialist Soviet Republic of Byelorussia (SSRB) was declared by the Bolsheviks on 1 January 1919 following the declaration of independence by the Belarusian Democratic Republic in March 1918. In 1922, the BSSR was one of the four founding members of the Soviet Union, together with the Ukrainian SSR, the Transcaucasian SFSR and the Russian Soviet Federative Socialist Republic (RSFSR). Byelorussia was one of several Soviet republics occupied by Nazi Germany during World War II.
Towards the final years of the Soviet Union's existence, the Supreme Soviet of Byelorussian SSR adopted the Declaration of State Sovereignty on 27 July 1990. On 15 August 1991, Stanislaŭ Šuškievič was elected as the country's first president. Ten days later on 25 August 1991, Byelorussian SSR declared its independence and renamed to the Republic of Belarus. The Soviet Union was dissolved four months later on December 26, 1991.