king of Castile and Leon from 1284 to 1295, second son of Alfonso X. Though ambitious and ruthless, he was also an able politician and a cultivated man.

In 1275 his elder brother, Fernando de la Cerda, was killed, leaving a son, Alfonso de la Cerda, heir to Alfonso X. Sancho, supported by the nobles and the military orders, sought recognition as heir instead of his nephew and took up arms against the king. After some years of strife he succeeded in making good his usurpation on his father's death (April 1284). Support for Alfonso de la Cerda came from factions at home and from France and Aragon. Sancho finally removed this threat by arranging a marriage (1291) between his eldest daughter, Isabel, and James II of Aragon. Despite these political troubles he succeeded in defeating an invasion of Andalusia by the king of Fès (1290). Sancho owed much to his ablest supporter, Lope Díaz de Haro, whom he killed in anger during an argument at Alfaro (1288). He also depended greatly on his warrior-queen, María de Molina, who served as regent for his son Ferdinand IV.

king of Pamplona (Navarre) from 1054 to 1076, son of García III (or IV).

Sancho had to contend with Castilian irredentism and Aragonese ambition. His act of persuading the Moorish king of Saragossa to become his vassal offended Alfonso VI of Castile, who invaded Pamplona (1074) and conspired with Sancho's brothers and magnates to assassinate him at Peñalén. The kingdom of Pamplona was then occupied by Sancho Ramírez of Aragon, with Navarrese approval.