Before the perestroika reforms of Mikhail Gorbachev that promoted a more liberal form of socialism, the formal ideology of the Communist Party of the Soviet Union (CPSU) was Marxism–Leninism, a form of socialism consisting of a centralised command economy with a vanguardist one-party state that aimed to realize the dictatorship of the proletariat. The Soviet Union's ideological commitment to achieving communism included the national communist development of socialism in one country and peaceful coexistence with capitalist countries while engaging in anti-imperialism to defend the international proletariat, combat the predominant prevailing global system of capitalism and promote the goals of Bolshevism. The state ideology of the Soviet Union – and thus Marxism–Leninism – derived and developed from the theories, policies, and political praxis of Karl Marx, Friedrich Engels, Vladimir Lenin, and Joseph Stalin.

Marxism–Leninism was the ideological basis for the Soviet Union. It explained and legitimized the CPSU's right to rule, while explaining its role as a vanguard party. For instance, the ideology explained that the CPSU's policies, even if they were unpopular, were correct because the party was enlightened. It was represented to be the only truth in Soviet society, and with it rejected the notion of multiple truths. In short, it was used to justify CPSU Leninism as being a means to an end. The relationship between ideology and decision-making was at best ambivalent, with most policy decisions taken in the light of the continued, permanent development of Marxism–Leninism, which, as the only truth, could not by its very nature become outdated.

Despite having evolved over the years, Marxism–Leninism had several central tenets. The main tenet was the party's status as sole ruling party. The 1977 Constitution referred to the party as the "leading and guiding force of Soviet society, and the nucleus of its political system, of all state and public organizations, is the Communist Party of the Soviet Union." State socialism was essential, and from Joseph Stalin until Mikhail Gorbachev official discourse considered private social and economic activity such as capitalism as "retarding" the development of Russian collective consciousness and of the Soviet economy. Gorbachev supported privatization to a degree, but based his policies on Vladimir Lenin's and Nikolai Bukharin's view on the New Economic Policy of the 1920s, and supported complete state ownership over the commanding heights of the economy. Unlike liberalism, Marxism–Leninism stressed not the importance of the individual, but rather the role of the individual as a member of a collective. Thus defined, individuals had only the right to freedom of expression if it safeguarded the interests of the collective. For instance, in the 1977 Constitution Marxism–Leninism it was stated that every person had the right to express their opinion, but that opinion could only be expressed if it was in accordance with the "general interests of Soviet society." In short, the number of rights granted to an individual was decided by the state, and could be taken away by the state as it saw fit. Soviet Marxism–Leninism justified nationalism, and the media portrayed every victory of the Soviet Union as a victory for the communist movement as a whole. In large parts, Soviet nationalism was based upon ethnic Russian nationalism. Marxism–Leninism stressed the importance of the worldwide conflict between capitalism and socialism, and the Soviet press talked about progressive and reactionary forces, while claiming that socialism was on the verge of victory; that the "correlations of forces" were in the Soviet Union's favour. Until its late years of the USSR, the ideology had professed state atheism, and party members were formerly not allowed to be religious. The state professed a belief in the feasibility of total communist mode of production, and all policies were seen as justifiable if it contributed to the Soviet Union's reaching that stage.

In Marxist philosophy, Leninism is the body of political theory for the democratic organisation of a revolutionary vanguard party, and the achievement of a dictatorship of the proletariat, as political prelude to the establishment of the socialist mode of production, developed by Lenin. Since Karl Marx barely, if ever wrote about how the socialist mode of production would look like or function, these tasks were left for later scholars like Lenin to solve. His main contribution to Marxist thought is the concept of the vanguard party of the working class. The vanguard party was conceived to be a highly-knit centralised organization which was led by intellectuals, rather than by the working class itself. The party was open only to a small number of the workers, the reason being that the workers in Russia still had not developed class consciousness and therefore needed to be educated to reach such a state. Lenin believed that the vanguard party could initiate policies in the name of the working class even if the working class did not support them, since the vanguard party would know what was best for the workers, since the party functionaries had attained consciousness.

Lenin, in light of the Marx's theory of the state (which views the state as an oppressive organ of the ruling class), had no qualms of forcing change upon the country. He viewed the dictatorship of the proletariat, in contrast to the dictatorship of the bourgeoisie, as the dictatorship of the majority. The repressive powers of the state were to be used to transform the country, and to strip of the former ruling class of their wealth. Lenin believed that the transition from the capitalist mode of production to the socialist mode of production would last for a long period. According to some authors, Leninism was by definition authoritarian. In contrast to Karl Marx, who believed that the socialist revolution would be composed of and led by the working class alone, Lenin argued that a socialist revolution did not necessarily need to be led by or composed of the working class alone, instead contending that a revolution needed to be led by the oppressed classes of society, which in the case of Russia was the peasant class.

Stalinism, while not an ideology per se, refers to Stalin's thoughts and policies. Stalin's introduction of the concept "Socialism in One Country" in 1924 was a major turning point in Soviet ideological discourse. The Soviet Union did not need a socialist world revolution to construct a socialist society, Stalin claimed. Four years later, Stalin initiated his "Second Revolution" with the introduction of state socialism and central planning. In the early-1930s, he initiated collectivization of Soviet agriculture, by de-privatizing agriculture, but not turning it under the responsibility of the state, per se, instead creating peasant cooperatives. With the initiation of his "Second Revolution", Stalin launched a "Cult of Lenin" and a cult of personality centered upon himself. For instance, the name of the city of Petrograd was changed to Leningrad, the town of Lenin's birth was renamed Ulyanov (Lenin's birth-name), the Order of Lenin became the highest state award and portraits of Lenin were hung up everywhere; in public squares, factories and offices etc. The increasing bureaucracy which followed after the introduction of a state socialist economy was at complete odds with the Marxist notion of "the withering away of the state". Stalin tried to explain the reasoning behind it at the 16th Congress (held in 1930);

We stand for the strengthening of the dictatorship of the proletariat, which represents the mightiest and most powerful authority of all forms of State that have ever existed. The highest development of the State power for the withering away of State power —this is the Marxian formula. Is this "contradictory"? Yes, it is "contradictory." But this contradiction springs from life itself and reflects completely Marxist dialectic.

The idea that the state would wither away was later abandoned by Stalin at the 18th Congress (held in 1939), in which he expressed confidence that the state would exist, even if the Soviet Union reached communism, as long as it was encircled by capitalism. Two key concepts were created in the latter half of his rule; the "two camp" theory and that of "capitalist encirclement". The threat of capitalism was used to strengthen Stalin's personal powers, and Soviet propaganda began making a direct link with Stalin and stability in society, claiming that the country would crumble without the leader. Stalin deviated greatly from classical Marxism when it came to "subjective factors", claiming that party members, whatever rank, had to profess fanatic adherence to the party's line and ideology, and that otherwise those policies would fail.