The October Revolution, officially known as the Great October Socialist Revolution under the Soviet Union, also known as the Bolshevik Revolution, was a revolution in Russia led by the Bolshevik Party of Vladimir Lenin that was a key moment in the larger Russian Revolution of 19171923. It was the second revolutionary change of government in Russia in 1917. It took place through an armed insurrection in Petrograd (now Saint Petersburg) on 7 November 1917 [O.S. 25 October]. It was the precipitating event of the Russian Civil War.

The October Revolution followed and capitalized on the February Revolution earlier that year, which had overthrown the Tsarist autocracy, resulting in a liberal provisional government. The provisional government had taken power after being proclaimed by Grand Duke Michael, Tsar Nicholas II's younger brother, who declined to take power after the Tsar stepped down. During this time, urban workers began to organize into councils (soviets) wherein revolutionaries criticized the provisional government and its actions. The provisional government remained unpopular, especially because it was continuing to fight in World War I, and had ruled with an iron fist throughout the summer (including killing hundreds of protesters in the July Days).

Events came to a head in the fall as the Directorate, led by the left-wing Socialist Revolutionary Party, controlled the government. The left-wing Bolsheviks were deeply unhappy with the government, and began spreading calls for a military uprising. On 10 October 1917 (O.S.; 23 October, N.S.), the Petrograd Soviet, led by Trotsky, voted to back a military uprising. On 24 October (O.S.; 6 November, N.S.) the government shut down numerous newspapers and closed the city of Petrograd in an attempt to forestall the revolution; minor armed skirmishes broke out. The next day a full scale uprising erupted as a fleet of Bolshevik sailors entered the harbor and tens of thousands of soldiers rose up in support of the Bolsheviks. Bolshevik Red Guards forces under the Military-Revolutionary Committee began the occupation of government buildings on 25 October (O.S.; 7 November, N.S.), 1917. The following day, the Winter Palace (the seat of the Provisional government located in Petrograd, then capital of Russia) was captured.

As the Revolution was not universally recognized, the country descended into the Russian Civil War, which would last until 1923 and ultimately lead to the creation of the Soviet Union in late 1922. The historiography of the event has varied. The victorious Soviet Union viewed it as a validation of their ideology, and the triumph of the worker over capitalism. During Soviet times, revolution day was a national holiday, marking its importance in the country's founding story. On the other hand, the Western Allies saw it as a totalitarian coup, which used the democratic Soviet councils only until they were no longer useful. The event inspired many cultural works, and ignited communist movements across Europe and globally. Many MarxistLeninist parties around the world still celebrate October Revolution Day.

Red Guards (Russian: Красная гвардия) were paramilitary volunteer formations consisting mainly of factory workers, peasants, cossacks and partially of soldiers and sailors for "protection of the soviet power". Red Guards were a transitional military force of the collapsing Imperial Russian Army and the base formations of Bolsheviks during the October Revolution and the first months of the Russian Civil War. Most of them were formed in the time frame of the Russian Revolution of 1917, and some of the units were reorganized into the Red Army during 1918. The Red Guards formations were organized across most of the former Russian Empire, including territories outside the contemporary Russian Federation such as Finland, Poland, Estonia, Ukraine, and others. They were not centralized and were formed by decision of a local political party and local soviet members. By fighting to protect and extend the power of the Soviets, they aided the creation of a new state that (according to its original conception) would give "all power to the soviets": the Soviet Union.