It existed from 31 March 1940 until it was made part of the Russian SFSR on 16 July 1956 as the Karelian Autonomous Soviet Socialist Republic.

The latter was created in territory ceded by Finland in the Winter War by the Moscow Peace Treaty, namely the Karelian Isthmus and Ladoga Karelia, including the cities of Viipuri and Sortavala.

In September 1944, the Karelian Isthmus with Vyborg (Viipuri) was transferred from the Karelo-Finnish SSR to the Leningrad Oblast of the RSFSR, but Ladoga Karelia remained a part of the republic.

This move can perhaps be explained in the context of the general post-war improvement of Finno-Soviet relations,[1] which also included such steps as the Soviets' return of the Porkkala Naval Base leased territory to full Finnish sovereignty (January 1956), and leasing Maly Vysotsky Island and the Soviet section of the Saimaa Canal (conquered by the USSR in 1940 and 1944) back to Finland (1963).

The state emblem of the Soviet Union had to be changed to reflect this, with one of the 16 ribbons symbolizing constituent republics (that in the Finnish language) removed.