Religion in the Union of Soviet Socialist Republics (USSR) was dominated by the fact that it became the first state to have as one objective of its official ideology the elimination of existing religion, and the prevention of future implanting of religious belief, with the goal of establishing state atheism (gosateizm). However, the main religions of pre-revolutionary Russia persisted throughout the entire Soviet period and religion was never officially outlawed. Christians belonged to various denominations: Orthodox, Catholic, Baptist and various other Protestant denominations. The majority of the Muslims in the Soviet Union were Sunni, with the notable exception of Azerbaijan, which was majority Shia. Judaism also had many followers. Other religions, practiced by a small number of believers, included Buddhism and Shamanism.

Marxist–Leninist atheism, also known as Marxist–Leninist scientific atheism, is the antireligious element of Marxism–Leninism. Based upon a dialectical-materialist understanding of humanity's place in nature, Marxist–Leninist atheism proposes that religion is the opium of the people; thus, Marxism–Leninism advocates atheism, rather than religious belief.