The Estonian Soviet Socialist Republic (Estonian: Eesti Nõukogude Sotsialistlik Vabariik, abbreviated as Eesti NSV or ENSV; Russian: Эстонская Советская Социалистическая Республика, Estonskaya Sovetskaya Sotsalisticheskaya Respublika, abbreviated as ЭССР, ESSR), often abbreviated as Estonian SSR or ESSR, was a republic of the Soviet Union, administered by and subordinated to the Government of the Soviet Union. The ESSR was initially established on the territory of the Republic of Estonia on July 21, 1940, following the invasion of Soviet troops on June 17, 1940 and the installation of a puppet government backed by the Soviet Union, which declared Estonia a Soviet state. The Estonian SSR was subsequently incorporated into the USSR on August 9, 1940. This territory was also occupied by Nazi Germany from 1941 to 1944.

Most countriesdid not recognise the incorporation of Estonia de jure and only recognised its Soviet government de facto or not at all. A number of these countries continued to recognize Estonian diplomats and consuls who still functioned in the name of their former governments. This policy of non-recognition gave rise to the principle of legal continuity, which held that de jure, Estonia remained an independent state under illegal occupation throughout the period 1940–91.