Spanish socialist leader, prominent during the Second Republic, of which he became prime minister soon after the outbreak of the civil war of 1936–39.

Largo Caballero worked in Madrid as a plasterer before joining the Spanish Socialist Workers' Party (Partido Socialista Obrero Español; PSOE) in 1894. He soon became an official in the party's trade union federation, the Unión General de Trabajadores (UGT), and rose to become chief lieutenant of the union's head, Pablo Iglesias (Iglesias, Pablo). Sentenced to life imprisonment for his part in the general strike of August 1917, he was released on his election to the Cortes (parliament) in 1918. In 1925 he succeeded Iglesias as head of the UGT. Largo Caballero cooperated with the government of dictator Miguel Primo de Rivera (Primo de Rivera, Miguel) (1923–30) in hopes of increasing the PSOE's strength and standing. Minister of labour from 1931 to 1933 in the second Spanish Republic (1931–39), he introduced progressive labour legislation.

After the general elections of 1933, which inaugurated a period of centre-right government, Largo Caballero moved further to the left, spoke increasingly of Socialist revolution, and supported the abortive uprising of October 1934.

In September 1936 Largo Caballero became prime minister and minister of defense. He attempted to tighten army discipline and endeavoured to secure respect for governmental authority in the Republican war zone. But an extreme-left uprising in Barcelona (May 3–10, 1937) was used by the communists to provoke a cabinet crisis, and he was forced to resign.

After his fall, Largo Caballero was politically isolated by the new government of Juan Negrín (Negrín López, Juan). In 1939 he went into exile in France. Arrested by the French police in 1939, Largo Caballero was later released and placed under house arrest. In 1943 he was arrested by the German Gestapo and interned at the Dachau concentration camp. He was freed by Polish troops and returned to Paris, where he died and was buried. In 1978 his body was moved to Madrid.