The Communist Party of the Soviet Union (CPSU), at some points known as the Russian Communist Party or All-Union Communist Party and sometimes referred to as the Soviet Communist Party (SCP), was the founding and ruling political party of the Soviet Union. The CPSU was the sole governing party of the Soviet Union until 1990 when the Congress of People's Deputies modified Article 6 of the 1977 Soviet Constitution, which had previously granted the CPSU a monopoly over the political system.

The party started in 1898 as the Russian Social Democratic Labour Party. In 1903, that party split into a Menshevik (minority) and Bolshevik (majority) faction; the latter, led by Vladimir Lenin, is the direct ancestor of the CPSU and is the party that seized power in the October Revolution of 1917. Its activities were suspended on Soviet territory 74 years later, on 29 August 1991, soon after a failed coup d'état by conservative CPSU leaders against the reforming Soviet president and party general secretary Mikhail Gorbachev.

The CPSU was a communist party based on democratic centralism. This principle, conceived by Lenin, entails democratic and open discussion of policy issues within the party, followed by the requirement of total unity in upholding the agreed policies. The highest body within the CPSU was the Party Congress, which convened every five years. When the Congress was not in session, the Central Committee was the highest body. Because the Central Committee met twice a year, most day-to-day duties and responsibilities were vested in the Politburo, (previously the Presidium), the Secretariat and the Orgburo (until 1952). The party leader was the head of government and held the office of either General Secretary, Premier or head of state, or two of the three offices concurrently, but never all three at the same time. The party leader was the de facto chairman of the CPSU Politburo and chief executive of the Soviet Union. The tension between the party and the state (Council of Ministers of the Soviet Union) for the shifting focus of power was never formally resolved.

After the founding of the Soviet Union in 1922, Lenin had introduced a mixed economy, commonly referred to as the New Economic Policy, which allowed for capitalist practices to resume under the Communist Party dictation in order to develop the necessary conditions for socialism to become a practical pursuit in the economically undeveloped country. In 1929, as Joseph Stalin became the leader of the party, Marxism–Leninism, a fusion of the original ideas of German philosopher and economic theorist Karl Marx, and Lenin, became formalized as the party's guiding ideology and would remain so throughout the rest of its existence. The party pursued state socialism, under which all industries were nationalized, and a command economy was implemented. After recovering from the Second World War, reforms were implemented which decentralized economic planning and liberalized Soviet society in general under Nikita Khrushchev. By 1980, various factors, including the continuing Cold War, and ongoing nuclear arms race with the United States and other Western European powers and unaddressed inefficiencies in the economy, led to stagnant economic growth under Alexei Kosygin, and further with Leonid Brezhnev and growing disillusionment. After the younger, vigorous Mikhail Gorbachev assumed leadership in 1985 (following two short-term elderly leaders, Yuri Andropov and Konstantin Chernenko, who quickly died in succession), rapid steps were taken to transform the tottering Soviet economic system in the direction of a market economy once again. Gorbachev and his allies envisioned the introduction of an economy similar to Lenin's earlier New Economic Policy through a program of "perestroika", or restructuring, but their reforms, along with the institution of free multi-candidate elections led to a decline in the party's power, and after the dissolution of the Soviet Union, the banning of the party by later last RSFSR President Boris Yeltsin and subsequent first President of an evolving democratic and free-market economy of the successor Russian Federation.

A number of causes contributed to CPSU's loss of control and the dissolution of the Soviet Union during the early 1990s. Some historians have written that Gorbachev's policy of "glasnost" (political openness) was the root cause, noting that it weakened the party's control over society. Gorbachev maintained that perestroika without glasnost was doomed to failure anyway. Others have blamed the economic stagnation and subsequent loss of faith by the general populace in communist ideology. In the final years of the CPSU's existence, the Communist Parties of the federal subjects of Russia were united into the Communist Party of the Russian Soviet Federative Socialist Republic (RSFSR). After the CPSU's demise, the Communist Parties of the Union Republics became independent and underwent various separate paths of reform. In Russia, the Communist Party of the Russian Federation emerged and has been regarded as the inheritor of the CPSU's old Bolshevik legacy into the present day.