The Republics of the Soviet Union or the Union Republics (Russian: союзные республики, soyuznye respubliki) of the Soviet Union were ethnically based administrative units that were subordinated directly to the Government of the Soviet Union. Historically a highly centralized state, the decentralization and democratization reforms during the era of Perestroika and Glasnost conducted by Mikhail Gorbachev led to the Dissolution of the USSR.

According to the Article 76 of the 1977 Soviet Constitution, the sovereign Soviet socialist states united to become 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".

According to the European Court of Human Rights, the United Nations Human Rights Council , the governments of the Baltic countries, the United States, and the European Union, the three Soviet Baltic republics (Estonian SSR, Latvian SSR, and Lithuanian SSR) were occupied by the Soviet Union in 1940 under the provisions of the 1939 Molotov-Ribbentrop Pact. The Russian government and state officials, however, maintain that the Soviet annexation of the Baltic states was legitimate.

In the final decades of its existence, the Soviet Union consisted of fifteen Soviet Socialist Republics (SSR). All of them were considered to be socialist republics, and all of them, with the exception of the Russian SFSR, had their own Communist parties, part of the Communist Party of the Soviet Union. All of the former Republics are now independent countries, with twelve of them (all except the Baltic states) being very loosely organized under the heading of the Commonwealth of Independent States.

Constitutionally, the Soviet Union was a federation. In accordance with provisions present in the Constitution (versions adopted in 1924, 1936 and 1977), each republic retained the right to secede from the USSR. Throughout the Cold War, this right was widely considered to be meaningless; however, the corresponding Article 72 of the 1977 Constitution was used in December 1991 to effectively dissolve the Soviet Union, when Russia, Ukraine, and Belarus seceded from the Union.

In practice, the USSR was a highly centralised entity from its creation in 1922 until the mid-1980s when political forces unleashed by reforms undertaken by Mikhail Gorbachev resulted in the loosening of central control and its ultimate collapse. Under the constitution adopted in 1936 and modified along the way until October 1977, the political foundation of the Soviet Union was formed by the Soviets (Councils) of People's Deputies. These existed at all levels of the administrative hierarchy, with the Soviet Union as a whole under the nominal control of the Supreme Soviet of the USSR, located in Moscow within the Russian Soviet Federative Socialist Republic.

Along with the state administrative hierarchy, there existed a parallel structure of party organizations, which allowed the Politburo to exercise large amounts of control over the republics. State administrative organs took direction from the parallel party organs, and appointments of all party and state officials required approval of the central organs of the party. General practice in the republics outside of Russia was that the head of state in a republic was a local official while the party general secretary was from outside the republic.

Each republic had its own unique set of state symbols: a flag, a coat of arms, and, with the exception of the Russian SFSR, an anthem. Every republic of the Soviet Union also was awarded with the Order of Lenin. Two (Ukraine and Belarus) were members of the United Nations General Assembly.