John I (August 24 1358 – 1390) (in Spanish: Juan I) was the king of Castile, was the son of Henry II and of his wife Joan, daughter of John Manuel of Villena , head of a younger branch of the royal house of Castile.

In the beginning of his reign he had to contend with the hostility of John of Gaunt, who claimed the crown by right of his wife Constance daughter of Peter 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 Beatrice of Portugal, daughter of Ferdinand of Portugal. 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. The 1383-1385 Crisis, a period of civil unrest and anarchy in Portugal, followed.

He was resisted by the national sentiment of the people, and was utterly defeated at the battle of Aljubarrota, on August 14, 1385. King John was killed at Alcalá on October 9, 1390 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.