The Lithuanian Soviet Socialist Republic (; , Litovskaya Sovetskaya Sotsialisticheskaya Respublika), also known as the Lithuanian SSR, was a republic of the Soviet Union. It existed from 1940 to 1990.
Established on 21 July 1940 as a puppet state, during World War II in the territory of the previously independent Republic of Lithuania after it had been occupied by the Soviet army on 16 June 1940, in conformity with the terms of the 23 August 1939 Molotov–Ribbentrop Pact.
Between 1941 and 1944, the German invasion of the Soviet Union caused its de facto dissolution. However, with the retreat of the Germans in 1944–1945, Soviet hegemony was re-established, and existed until 1990.