Diego Martínez Barrio (Seville, 1882 – Paris, 1965) was a Spanish politician during the Second Spanish Republic and briefly held the position of Prime Minister of Spain after the resignation of Santiago Casares Quiroga, on July 19, 1936, three days after the beginning of the Spanish Civil War.

A member of the Radical Republican Party, he was the Minister of Government with Alejandro Lerroux, though later he left the party due to his dissatisfaction with the politics of Lerroux.

Barrio consequently founded and led the Republican Union Party and participated in the Spanish Popular Front, elected to government in 1936. He led the integration of the Republican Union Party into the Popular Front. He was elected the speaker of the Cortes (Spanish Parliament), and was later appointed prime minister by Manuel Azaña after the resignation of Santiago Casares Quiroga. He fled the country after Francisco Franco came to power in 1939.

After the fall of the Republic he went into exile, first to France and then to Mexico where in 1945 he was designated President of the Republic in exile. Barrio finally returned to Paris, where he died in 1965. Thirty five years later, in 2000, his remains were moved to Seville, his hometown.