Fadrique Alfonso of Castile

Fadrique Alfonso of Castile (1334–1358) was an illegitimate son of Alfonso XI and Eleanor of Guzmán and twin brother of Henry of Trastámara. While still a child in 1342 he succeeded his maternal uncle as master of the Order of Santiago. He held Haro and served as adelantado mayor of the Andalusian frontier.

In 1353 he escorted Peter I's bride Blanche of Bourbon back from France, and gossip later said he had been too close to her on the road. Joining his half-brothers' revolt in the war with Aragon, he was murdered at Seville in 1358 on Peter's orders.

Chroniclers later gave him the story of being thrown from a window, which really belongs to another victim, Prince John of Aragon. The Enríquez admirals descended from him.