The Republics of the Soviet Union or the Union Republics (Russian: Сою́зные Респу́блики, tr. Soyúznye Respúbliki) of the Soviet Union were ethnically based proto-states that were subordinated directly to the Government of the Soviet Union. For most of its history, the Soviet Union was a highly centralized state; the decentralization reforms during the era of Perestroika ("Restructuring") and Glasnost ("Openness") conducted by Mikhail Gorbachev led to the dissolution of the Soviet Union in 1991.
According to Article 76 of the 1977 Soviet Constitution, a Union Republic was a sovereign Soviet socialist state that had united with other Soviet Republics in the Union of Soviet Socialist Republics. Article 81 of the Constitution stated that "the sovereign rights of Union Republics shall be safeguarded by the USSR".
In the final decades of its existence, the Soviet Union officially consisted of fifteen Soviet Socialist Republics (SSRs). All of them, with the exception of the Russian Federation (until 1990), had their own local party chapters of the All-Union Communist Party.
Outside the territory of the Russian Federation, the republics were constituted mostly in lands that had formerly belonged to the Russian Empire and had been acquired by it between the 1700 Great Northern War and the Anglo-Russian Convention of 1907.
In 1944, amendments to the All-Union Constitution allowed for separate branches of the Red Army for each Soviet Republic. They also allowed for Republic-level commissariats for foreign affairs and defense, allowing them to be recognized as de jure independent states in international law. This allowed for two Soviet Republics, Ukraine and Byelorussia, (as well as the USSR as a whole) to join the United Nations General Assembly as founding members in 1945.