- (1285-1312) Son of Sancho IV of Castile and María de Molina. During his childhood, his mother ruled as regent.
- Married Constança of Portugal and Aragon (1302) by whom he had the future Alfonso XI of Castile ( 1311-1350). Constança was the daughter of King Dinis of Portugal and Isabel of Aragon.
- Fernando IV of Castile was grandfather to Pedro of Castile. [Pedro, ch. 15: 60]