The Estonian Soviet Socialist Republic, also known as the Estonian SSR, Soviet Estonia, or simply Estonia, was a union republic and an ethnically based administrative subdivision of the former Soviet Union (USSR) covering the occupied and annexed territory of Estonia in 1940–1941 and 1944–1991. The Estonian SSR was nominally established to replace the until then independent Republic of Estonia on 21 July 1940, a month after the 16–17 June 1940 Soviet military invasion and occupation of the country during World War II. After the installation of a Stalinist government which, backed by the occupying Soviet Red Army, declared Estonia a Soviet constituency, the Estonian SSR was subsequently incorporated into the Soviet Union as a "union republic" on 6 August 1940. Estonia was occupied by Nazi Germany in 1941, and administered as a part of Reichskommissariat Ostland until it was reconquered by the USSR in 1944.

The majority of the world's countries did not recognize the incorporation of Estonia into the Soviet Union de jure and only recognized its Soviet administration de facto or not at all. A number of countries continued to recognize Estonian diplomats and consuls who still functioned in the name of their former government. This policy of non-recognition gave rise to the principle of legal continuity, which held that de jure, Estonia remained an independent state under occupation throughout the period 1940–1991.

On 16 November 1988, Estonia became the first of the then Soviet-controlled countries to declare state sovereignty from the central government in Moscow. On 30 March 1990, the newly elected parliament declared that the Republic of Estonia had been illegally occupied since 1940, and formally announced a transitional period for the restoration of the country's full independence. Subsequently, on 8 May 1990, the use of all Soviet symbols in Estonia was officially abolished and from the "Estonian Soviet Socialist Republic" two words, "Soviet" and "Socialist", were dropped, effectively returning to the name "Republic of Estonia". The parliament of Estonia declared the re-establishment of full independence on 20 August 1991. The Soviet Union formally recognized the independence of Estonia on 6 September 1991.