Juan Domingo Astigarrabía Andonegui was born in San Sebastián, the capital of Gipuzkoa, on 20 November 1901.

As a young man he joined the Federación Vasco-Navarra of the Partido Comunista Español (PCE, Spanish Communist Party), and with the Zapirain brothers made the first contacts in 1924–25 in the construction industry.

[1] At the time the Second Spanish Republic (1931–1939) was created Astigarrabía was one of the group of Basque communists that headed the trade union movement in San Sebastián and Pasaia.

He led a march of fishermen from Pasaia to the capital, which resulted in several deaths by gunfire from the security forces.

[2] In 1932 the Spanish Communist Party made a major change in direction when it abandoned the Comintern slogan "Workers' and Peasants' Government" and adopted "Defense of the Republic".

[1] In June 1935 he was elected first secretary-general of the Communist Party of the Basque Country (Euskadiko Partidu Komunista, EPK) at its founding conference.

[6] With the outbreak of the Spanish Civil War in July 1936 Astigarrabía and the other Basque communist leaders, Jesús Monzón and Ramón Ormazábal, became isolated in the North.

Although the Basques had the right for self-determination, this could only be achieved after the successful defense of the Spanish Republic against the Fascists and the victory of the popular revolution.

[8] Ramón Ormazabal accused him and Jesús Larrañaga of causing the fall of Biscay through "appeasement" and nationalism.