Grigory Yevseevich Zinoviev (Григо́рий Евсе́евич Зино́вьев, real name Ovsel Gershon Aronov Radomyslsky (Радомысльский), also known as Hirsch Apfelbaum), (September 23 [September 11, Old Style], 1883 - August 25, 1936) was a Bolshevik revolutionary and a Soviet Communist politician.

Zinoviev was a member of the Bolshevik faction from its creation in 1903, and one of Lenin's closest associates. He returned to Russia in 1917 in a sealed train with Lenin, but later lost favor due to his opposition to the Bolshevik seizure of power in October. When it came to real action, Zinoviev shrank from the proposed revolutionary coup and on October 10, 1917, he and Kamenev were the only two Central Committee members to vote against Lenin on the issue of staging the armed action which was to place the Bolsheviks in power.

Zinoviev took virtually no part in the October revolution, and Lenin did not forget his faint-heartedness. However, Zinoviev soon returned to the fold, and became a member of the powerful Politburo from 1919, as well as the head of the Comintern.

He was one of the most powerful figures in the USSR after Lenin's death in 1924. Initially, he, along with his close associate Lev Kamenev, formed part of a ruling triumvirate in the Communist Party with Josef Stalin, and played a key role in the marginalisation of Leon Trotsky. Stalin began to sideline Zinoviev once Trotsky was subdued and he had consolidated his powerbase in the party. Zinoviev and Kamenev were removed from positions of influence within the party and government.

As part of Stalin's Great Purges, Zinoviev, Kamenev, and others were arrested in 1935 and charged with being involved in the assassination of Sergei Kirov, a murder possibly staged by Stalin himself. Found guilty, Zinoviev was sentenced to 10 years' imprisonment. The following year, he was charged with forming a terrorist organization to kill Joseph Stalin and other leaders of the government. Along with Lev Kamenev, he was found guilty and executed on August 25, 1936.

Zinoviev is remembered in Britain as the putative author of the 'Zinoviev Letter' which caused a sensation when published on October 25, 1924, four days before a general election. The letter called on British Communists to prepare for revolution. It is now generally accepted to have been a forgery.