The All-Russian Extraordinary Commission (Russian: Всероссийская чрезвычайная комиссия, romanized: Vserossiyskaya chrezvychaynaya komissiya, IPA: [fsʲɪrɐˈsʲijskəjə tɕrʲɪzvɨˈtɕæjnəjə kɐˈmʲisʲɪjə]), abbreviated as VChK (Russian: ВЧК, IPA: [vɛ tɕe ˈka]), and commonly known as the Cheka (Russian: ЧК, IPA: [tɕɪˈka]), was the first Soviet secret police organization. It was established on 20 December [O.S. 7 December] 1917 by the Council of People's Commissars of the Russian SFSR, and was led by Felix Dzerzhinsky. By the end of the Russian Civil War in 1922, the Cheka had at least 200,000 personnel.

Ostensibly created to protect the October Revolution from "class enemies" such as the bourgeoisie and members of the clergy, the Cheka soon became a tool of repression wielded against all political opponents of the Bolshevik regime. The organization had responsibility for counterintelligence, oversight of the loyalty of the Red Army, and protection of the country's borders, as well as the collection of human and technical intelligence. At the direction of Vladimir Lenin, the Cheka performed mass arrests, imprisonments, torture, and executions without trial in what came to be known as the "Red Terror". It policed the Gulag system of labor camps, conducted requisitions of food, and put down rebellions by workers and peasants. The Cheka was responsible for executing at least 50,000 to as many as 200,000 people, though estimates vary widely.

The Cheka, the first in a long succession of Soviet secret police agencies, established the security service as a major player in Soviet politics. It was dissolved in February 1922, and succeeded by the State Political Directorate (GPU). Throughout the Soviet era, members of the secret police were referred to as "Chekists".

The official designation was All-Russian Extraordinary (or Emergency) Commission for Combating Counter-Revolution and Sabotage under the Council of People's Commissars of the RSFSR (Russian: Всероссийская чрезвычайная комиссия по борьбе с контрреволюцией и саботажем при Совете народных комиссаров РСФСР, Vserossiyskaya chrezvychaynaya komissiya po borbe s kontrrevolyutsiyey i sabotazhem pri Sovete narodnykh komisarov RSFSR).

In 1918, its name was changed, becoming All-Russian Extraordinary Commission for Combating Counter-Revolution, Profiteering and Corruption.

A member of Cheka was called a chekist (Russian: чеки́ст, romanized: chekíst, IPA: [t͡ɕɪˈkʲist] ^ⓘ). Also, the term chekist often referred to Soviet secret police throughout the Soviet period, despite official name changes over time. In The Gulag Archipelago, Alexander Solzhenitsyn recalls that zeks in the labor camps used old chekist as a mark of special esteem for particularly experienced camp administrators. The term is still found in use in Russia today (for example, President Vladimir Putin has been referred to in the Russian media as a chekist due to his career in the KGB and as head of the KGB's successor, FSB).

The Chekists commonly dressed in black leather, including long flowing coats, reportedly after being issued such distinctive coats early in their existence. Western communists adopted this clothing fashion. The Chekists also often carried with them Greek-style worry beads made of amber, which had become "fashionable among high officials during the time of the 'cleansing'".

In 1921, the Troops for the Internal Defense of the Republic (a branch of the Cheka) numbered at least 200,000. These troops policed labor camps, ran the Gulag system, conducted requisitions of food, and subjected political opponents to secret arrest, detention, torture and summary execution. They also put down rebellions and riots by workers or peasants, and mutinies in the desertion-plagued Red Army.