Trotskyism is the political ideology and branch of Marxism developed by Russian revolutionary Leon Trotsky and some other members of the Left Opposition and Fourth International. Trotsky described himself as an orthodox Marxist, a revolutionary Marxist, and Bolshevik–Leninist, a follower of Marx, Engels, and Vladimir Lenin, Karl Liebknecht, and Rosa Luxemburg. He supported founding a vanguard party of the proletariat, proletarian internationalism, and a dictatorship of the proletariat (as opposed to the "dictatorship of the bourgeoisie", which Marxists argue defines capitalism) based on working-class self-emancipation and mass democracy. Trotskyists are critical of Stalinism as they oppose Joseph Stalin's theory of socialism in one country in favour of Trotsky's theory of permanent revolution. Trotskyists criticize the bureaucracy and anti-democratic current developed in the Soviet Union under Stalin.

Vladimir Lenin and Trotsky, despite their ideological disputes, were close personally prior to the London congress of social democrats in 1903 and during the First World War. Lenin and Trotsky were close ideologically and personally during the Russian Revolution and its aftermath, and Trotskyists and some others call Trotsky its "co-leader". Trotsky was the Red Army's paramount leader in the Revolutionary period's direct aftermath. Trotsky initially opposed some aspects of Leninism but eventually concluded that unity between the Mensheviks and Bolsheviks was impossible and joined the Bolsheviks. Trotsky played a leading role with Lenin in the October Revolution. Assessing Trotsky, Lenin wrote: "Trotsky long ago said that unification is impossible. Trotsky understood this and from that time on, there has been no better Bolshevik."

In 1927, Trotsky was purged from the Communist Party and Soviet politics. In October, by order of Stalin, Trotsky was removed from power and, in November, expelled from the All-Union Communist Party (Bolsheviks). He was exiled to Alma-Ata (now Almaty) in January 1928 and then expelled from the Soviet Union in February 1929. As the head of the Fourth International, Trotsky continued in exile to oppose what he termed the degenerated workers' state in the Soviet Union. On 20 August 1940, Trotsky was attacked in Mexico City by Ramón Mercader, a Spanish-born NKVD agent, and died the next day in a hospital. His murder is considered a political assassination. Almost all Trotskyists within the All-Union Communist Party (Bolsheviks) were executed in the Great Purges of 1937–1938, effectively removing all of Trotsky's internal influence in the Soviet Union. Nikita Khrushchev had come to power as head of the Communist Party in Ukraine, signing lists of other Trotskyists to be executed. Trotsky and the party of Trotskyists were still recognized as enemies of the USSR during Khrushchev's rule of the Soviet Union from 1956.

Trotsky's Fourth International was established in the French Third Republic in 1938 when Trotskyists argued that the Comintern or Third International had become irretrievably "lost to Stalinism" and thus incapable of leading the international working class to political power. In contemporary English language usage, an advocate of Trotsky's ideas is often called a "Trotskyist". A Trotskyist may be called a "Trotskyite" or "Trot", especially by a critic of Trotskyism.