- (1358-1390) Son of Henrique II and Juana Manuel of Castile.
- Married Leonor of Aragon, daughter of Pedro IV of Aragon, 'the Cerimonious', and Eleonora of Sicily. From this marriage were born the future Enrique III of Castile and Fernando I 'the Just' of Aragon.
- His second wife was Beatriz of Portugal, daughter of King Fernando and Queen Leonor Teles of Portugal. The marriage was agreed through the Treaty of Salvaterra (1383) and intended to bring to an end the hostilities with Portugal. King Fernando's objective was to guarantee the survival of Portugal as an independent kingdom to be inherited by any offspring from his daughter Beatriz and Juan I of Castile, with his widow Queen Leonor Teles as Regent during the child's minority. Juan I, however, had other intentions and shortly afterwards, on the death of his Portuguese father-in-law, invaded Portugal, claiming the throne on behalf of his wife Beatriz.
- Juan I was a main participant in the 1383-1385 dynastic crisis in Portugal. His invasion of the country to claim the Portuguese crown caused an uprising to flare up in Portugal led by João, Master de Avis, an illegitimate son of King Pedro of Portugal, and supported by Nuno Álvares Pereira, the military leader of his faction, the lower Portuguese nobility and the common people, namely the city dwellers of Lisbon, who endured a four-month long siege. Juan I had to abandon the siege when the plague spread through his camp, infecting even Queen Beatriz. He made another dramatic attempt to conquer the Portuguese crown at the Battle of Aljubarrota (1385), but was thoroughly defeated despite leading an army vastly superior to the Portuguese.
- Juan I also had a long period of hostilities with John of Gaunt, who invaded Castile with the assistance of João I of Portugal, claiming the Castilian throne on behalf of his wife Constanza. She was the daughter of Pedro of Castile who had been killed by Juan I's father, Enrique de Trastámara, at Montiel (1369), during the latter's struggle for the Castilian throne. These hostilities eventually came to an end through the Treaty of Bayonne (1388) and the marriage of Juan I's son, the future Enrique III of Castile, to Catherine, John of Gaunt's daughter by Constanza of Castile. Through this treaty, the heirs of Pedro of Castile recovered their inheritance rights to the crown. Not to be neglected is also the fact that John of Gaunt received a substantial payment from Juan I, and he could extricate himself honorably from the conflict, as it was becoming increasingly difficult to obtain the funds and the men necessary to wage a war on the Iberian Peninsula.
Juan I de Castela