Diego Martínez Barrio (1882 - 1965) was a Spanish socialist politician.

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

Barrio died in 1965.