The Fourth International (FI) was a political international established in France in 1938 by Leon Trotsky and his supporters, having been expelled from the Soviet Union and the Communist International (also known as Comintern or the Third International).

There is no longer a single, centralized cohesive Fourth International. Throughout most of its existence and history, the Fourth International was hunted by agents of the NKVD, subjected to political repression by countries such as France and the United States, and by supporters of the Soviet Union. The Fourth International struggled to maintain contact under these conditions of crackdowns and repression during World War II due to the fact that subsequent proletarian uprisings were often under the influence of Soviet-aligned pro-Stalin parties and militant nationalist groups, leading to defeats for the Fourth International and the Trotskyists, who subsequently never managed to obtain meaningful influence.

Despite this, many parts of the world, including Latin America, Europe and Asia, continue to have large Trotskyist groupings who are attracted to its anti-Stalinist positions and its defense of proletarian internationalism. Several of these groups carry the label "Fourth Internationalist" either in their organization's name, important political position documents, or both. In line with Trotskyist theory and thought, the Fourth International tended to view the Soviet Union as a degenerated workers' state. However, although it regarded its own ideas as more advanced and thus superior to those of the Third International, it did not actively push for the Comintern's destruction. The current incarnation of the Fourth International does not operate as a cohesive entity in the manner of its predecessors.

The Fourth International suffered a major split in 1940 and an even more significant schism in 1953. A partial reunification of the schismatic factions occurred in 1963, but the organization never recovered sufficiently, and it failed to re-emerge as a single transnational grouping. The response of Trotskyists to such a situation has been in the form of forming multiple Internationals across the world, with some divided over which particular organization represents the true legacy and political continuity of the historical Fourth International.

Trotskyists regard themselves as working in opposition to both capitalism and Stalinism. Trotsky advocated proletarian revolution as set out in his theory of "permanent revolution", and believed that a workers' state would not be able to hold out against the pressures of a hostile capitalist world unless socialist revolutions quickly took hold in other countries as well. This theory was advanced in opposition to the view held by the Stalinists that "Socialism in One Country" could be built in the Soviet Union alone. Furthermore, Trotsky and his supporters harshly criticized the increasingly totalitarian nature of Joseph Stalin's rule. They argued that socialism without democracy is impossible. Thus, faced with the increasing lack of democracy in the Soviet Union, they concluded that it was no longer a socialist workers' state, but a degenerated workers' state.

In the Transitional Program, which was drafted during the founding congress of the Fourth International, Trotsky called for the legalization of the Soviet parties and worker's control of production.

Trotsky and his supporters had been organised since 1923 as the Left Opposition. They opposed the bureaucratization of the Soviet Union, which they analysed as being partly caused by the poverty and isolation of the Soviet economy. Stalin's theory of socialism in one country was developed in 1924 as an opposition to Trotsky's theory of permanent revolution, which argued that capitalism was a world system and required a world revolution to replace it with socialism. Prior to 1924, the Bolsheviks' international perspective had been guided by Trotsky's position. Trotsky argued that Stalin's theory represented the interests of bureaucratic elements in direct opposition to the working class.

Eventually, Trotsky was sent into internal exile and his supporters were jailed. However, the Left Opposition continued to work in secret within the Soviet Union. Trotsky was exiled to Turkey in 1928. He moved from there to France, Norway and finally to Mexico. He was assassinated on Stalin's orders in Mexico in August 1940.