king of Castile and Leon from 1312, who succeeded his father, Ferdinand IV, when he was only a year old.

His minority was marked by violent strife between factions of nobles, but when he came of age, in 1325, he restored order with unprecedented vigour. He gave new powers to the municipalities and to the Cortes, in exchange for their support against the nobles, and furthered the power of the crown by choosing officials without aristocratic affiliations. He then turned his attention to the Marinid kings of Morocco, who had seized Gibraltar and routed the Castilian fleet at Algeciras in 1340. With the Portuguese, he defeated the invaders at Río Salado (Río Salado, Battle of) in 1340 and recaptured Algeciras in 1344.

Alfonso XI promulgated important administrative and legal reforms in the ordinances of Alcalá de Henares in 1348. Alfonso was assiduously courted by both France and England, who wished for an alliance that would give them the support of his powerful fleet, but he avoided committing himself to either party.