John I (b. 1358, Épila, Castile, d. 1390, Alcalá de Henares), king of Castile from 1379 to 1390, son of Henry II, founder of the dynasty of Trastámara.
In the beginning of his reign John had to contend with the hostility of John of Gaunt, who claimed the crown by right of his wife Constance, daughter of Peter I the Cruel. The king of Castile finally bought off the claim of his English competitor by arranging a marriage between his son Henry and Catherine, daughter of John of Gaunt, in 1387. Before this date he had been engaged in hostilities with Portugal, which was in alliance with John of Gaunt. His first quarrel with Portugal was settled by his marriage, in 1382, with Beatriz, daughter of the Portuguese king Ferdinand. On the death of his father-in-law in 1383, John endeavoured to enforce the claims of his wife, Ferdinand's only child, to the crown of Portugal. He was resisted by the national sentiment of the people and was utterly defeated at the Battle of Aljubarrota (Aug. 14, 1385).
King John was killed at Alcalá five years later by the fall of his horse, while he was riding in a fantasia with some of the light horsemen known as the farfanes, who were mounted and equipped in the Arab style.