Julio Álvarez del Vayo (Villaviciosa de Odón, Community of Madrid, 1890 – Geneva, Switzerland, 1975) was a Spanish Socialist politician, journalist and writer.
Álvarez studied Law at the Universities of Madrid and Valladolid and he did postdoctoral work at the London School of Economics. He joined the Spanish Socialist Workers' Party (PSOE) at a very young age and he opposed to the collaboration of that party with the dictatorship of Primo de Rivera (1923–1930). He wrote for the newspapers La Nación of Argentina, El Liberal and El Sol of Spain, and The Guardian of Britain. He visited the United States, the European fronts during the First World War and the Soviet Union as a journalist. In 1930 he conspired for an armed uprising against the Monarchy. When the Second Republic was proclaimed he was appointed ambassador to Mexico and to the Soviet Union, and later he was elected a member of the Parliament. He followed the PSOE's revolutionary wing led by Largo Caballero.