Ferdinand I, 1345–83, king of Portugal (1367–83), son and successor of Peter I. His ambitions and his private life plunged the realm into disaster, although during his reign agricultural reform was achieved and Portuguese commercial power grew. Ferdinand's desire for the throne of Castile involved him in three wars with Castile. The first (1369–71) ended with Ferdinand's promise to marry Leonor, daughter of Henry II of Castile. Instead he fell in love with a Portuguese noblewoman, Leonor Teles, and after securing a dubious annulment of her earlier marriage, made her his queen. Ferdinand then allied (1372) himself with John of Gaunt and waged new war against Henry II, which led to a Castilian siege of Lisbon (1373) and a humiliating peace. After John I succeeded to the throne of Castile, Ferdinand, under the influence of his wife and her lover (the conde de Ourém), resumed the English alliance and engaged (1381–82) in a third humiliating war with Castile. It was concluded by the marriage of John with Ferdinand's daughter and heiress, Beatrice. Portugal would thus have gone to Castile on Ferdinand's death, but a national revolution gave the throne to Ferdinand's half-brother, John I.

Ferdinand I, 1379?–1416, king of Aragón and Sicily and count of Barcelona (1412–16), second son of John I of Castile; nephew and successor of Martin of Aragón. In 1406, Ferdinand became regent of Castile during the minority of his nephew, John II. He captured (1410) Antequera from the Moors and claimed the vacant throne of Aragón in the same year. Finally chosen king in 1412, he defeated (1413) his chief rival for the throne and suppressed revolts in Sicily and Sardinia. In 1415 he met Holy Roman Emperor Sigismund at Perpignan and was obliged to agree to the deposition of Antipope Benedict XIII (see Luna, Pedro de). Ferdinand was succeeded by his son, Alfonso V.

Ferdinand I or Ferdinand the Great, d. 1065, Spanish king of Castile (1035–65) and León (1037–65). He inherited Castile from his father, Sancho III of Navarre, conquered León, and took parts of Navarre from his brother García. Ferdinand fought successfully against the Moors and reduced to vassalage the Moorish kings of Zaragoza, Badajoz, Seville, and Toledo. At the Council of Coyanza (1050) he confirmed the laws of Alfonso V and introduced church reforms. He divided his kingdom among his sons: Castile went to Sancho II, León to Alfonso VI, and Galicia to García.