John I of Castile was a king who ruled from 1379 to 1390. He was the son of Henry II of Castile and Juana Manuel, and he became king after the death of his half-brother, Peter of Castile. His reign was marked by conflicts with the nobility and struggles to maintain control over his kingdom. During his rule, John I faced challenges from both internal factions and external threats, including the Kingdom of Aragon. He is also known for his marriage to Beatrice of Portugal, which strengthened ties between Castile and Portugal. His reign ended with his death in 1390.