The Left Opposition was a faction within the Russian Communist Party (Bolsheviks) from 1923 to 1927 headed de facto by Leon Trotsky. The Left Opposition was formed by Trotsky to mount a struggle against the bureaucratic degeneration that began within the party leadership headed by Stalin during the serious illness of the Bolshevik founder Vladimir Lenin. The degeneration intensified after Lenin's death in January 1924.

Lev Davidovich Bronstein, better known as Leon Trotsky, was a Russian revolutionary, Soviet politician, journalist, and political theorist. He was a central figure in the 1905 Revolution, October Revolution, Russian Civil War, and the establishment of the Soviet Union. Alongside Vladimir Lenin, Trotsky was widely considered the most prominent Soviet figure and was de facto second-in-command during the early years of the Russian Soviet Republic. Ideologically a Marxist and a Leninist, his thought and writings inspired a school of Marxism known as Trotskyism.