Viriatos, named after the Lusitanian leader Viriathus, was the generic name given to Portuguese volunteers who fought with the Nationalists in the Spanish Civil War. In the first weeks of the war the Portuguese Army tried to form a Viriatos Legion to aid the right-wing insurgents in Spain. The Legion was disbanded before any recruitment drive could take place after pro-Republican incidents in Portugal convinced the government that direct intervention on the side of the Nationalists could cause further unrest. Due to the widespread publicity given to the Viriatos Legion all Portuguese volunteers who subsequently enlisted directly in the Spanish Foreign Legion and Carlist militias were known as Viriatos.
According to Beevor, 12,000 Portuguese participated. The historian Christopher Othen claims the number was closer to 8,000. A Portuguese Military Observation Mission with members drawn from all three branches of the Portuguese military was present in Spain from 1937 onwards with the dual objectives of protecting the interests of Portuguese foreign volunteers and collating information on the lessons learned in the civil war. Although the Mission officially had a non-combatant role its airforce contingent took part in combat missions.