1977 Constitution of the Soviet Union in the context of "Presidium of the Supreme Soviet"

In the Soviet Union, a Union Republic (Russian: Сою́зная Респу́блика, romanized: Soyúznaya Respúblika) or unofficially a Republic of the USSR was a constituent federated political entity with a system of government called a Soviet republic, which was officially defined in the 1977 constitution as "a sovereign Soviet socialist state which has united with the other Soviet republics to form the Union of Soviet Socialist Republics" and whose sovereignty is limited by membership in the Union. As a result of its status as a sovereign state, the Union Republic de jure had the right to enter into relations with foreign states, conclude treaties with them and exchange diplomatic and consular representatives and participate in the activities of international organizations (including membership in international organizations). The Union Republics were perceived as national-based administrative units of the Union of Soviet Socialist Republics (USSR).

The Soviet Union was formed in 1922 by a treaty between the Soviet republics of Byelorussia, Russian SFSR (RSFSR), Transcaucasian Federation, and Ukraine, by which they became its constituent republics of the Union of Soviet Socialist Republics (Soviet Union). For most of its history, the USSR was a one-party state led by the Communist Party of the Soviet Union. Key functions of the USSR were highly centralized in Moscow until its final years, despite its nominal structure as a federation of republics; the light decentralization reforms during the era of perestroika (reconstruction) and glasnost (voice-ness, as in freedom of speech) conducted by Mikhail Gorbachev as part of the Helsinki Accords are cited as one of the factors which led to the dissolution of the USSR in 1991 as a result of the Cold War and the creation of the Commonwealth of Independent States.

The Supreme Soviet of the Union of Soviet Socialist Republics (SSUSSR) was the highest organ of state authority of the Soviet Union from 1936 to 1991. Based on the principle of unified power, it was the only branch of government in the Soviet state, and headed the unified state apparatus.

Prior to 1936, the Congress of Soviets was the highest organ of state authority. During 1989–1991 a similar, but not identical organ acted as the highest organ of state authority. The Supreme Soviet appointed the Council of Ministers, the Supreme Court, and the Procurator General of the Soviet Union as well as elected the Presidium which served as the Soviet Union's collective head of state under both the 1936 and 1977 Soviet Constitutions.

The Government of the Union of Soviet Socialist Republics (USSR) was the executive and administrative organ of the highest body of state authority, the All-Union Supreme Soviet. It was formed on 30 December 1922 and abolished on 26 December 1991. The government was headed by a chairman, most commonly referred to as the premier of the Soviet Union, and several deputy chairmen throughout its existence. The Communist Party of the Soviet Union (CPSU), as "The leading and guiding force of Soviet society and the nucleus of its political system" per Article 6 of the state constitution, controlled the government by holding a two-thirds majority in the All-Union Supreme Soviet. The government underwent several name changes throughout its history, and was known as the Council of People's Commissars from 1922 to 1946, the Council of Ministers from 1946 to 1991, the Cabinet of Ministers from January to August 1991 and the Committee on the Operational Management of the National Economy from August to December 1991.

The government chairman was nominated by the Central Committee of the Communist Party of the Soviet Union (CPSU) and elected by delegates at the first plenary session of a newly elected Supreme Soviet of the Soviet Union. Certain governments, such as Ryzhkov's second government, had more than 100 government ministers, serving as first deputy premiers, deputy premiers, government ministers or heads of state committees/commissions; they were chosen by the premier and confirmed by the Supreme Soviet. The Government of the Soviet Union exercised its executive powers in conformity with the constitution of the Soviet Union and legislation enacted by the Supreme Soviet. The first government was led by Vladimir Lenin, and the last government was led by Valentin Pavlov.

The Constitution of the Soviet Union recognised the Presidium of the Supreme Soviet (between 1938 and 1989) and the earlier Central Executive Committee (CEC) of the Congress of Soviets (between 1922 and 1938) as the highest organs of state authority in the Union of Soviet Socialist Republics (USSR) between legislative sessions. Under the 1924, 1936 and 1977 Soviet Constitutions these bodies served as the collective head of state of the Soviet Union. The chairman of these bodies personally performed the largely ceremonial functions assigned to a single head of state but was provided little real power by the constitution.

The Soviet Union was established in 1922. However, the country's first constitution was only adopted in 1924. Before that time, the 1918 Constitution of the Russian Soviet Federative Socialist Republic functioned as the constitution of the USSR. According to the 1918 Constitution, the All-Russian Central Executive Committee (CEC), whose chairman was head of state, had the power to determine what matters of income and taxation would go to the state budget and what would go to the local soviets. The CEC could also limit taxes. In periods between convocations of the Congress of Soviets the CEC held supreme power. In between sessions of the Congress of Soviets the CEC was responsible for all the affairs of the Congress of Soviets. The CEC and the Congress of Soviets was replaced by the Presidium and the Supreme Soviet respectively by several amendments to the 1936 Constitution in 1938.