 Born on 13 August 1311 in Salamanca, he was the son of King Ferdinand IV of Castile and Constance of Portugal.His father died when Alfonso was one year old.His grandmother, María de Molina, his mother Constance, his granduncle "Infante" John of Castile, Lord of Valencia de Campos, son of King Alfonso X of Castile and uncle "Infante" Peter of Castile, Lord of Cameros, son of King Sancho IV assumed the regency.