Alfonso was born in Naples, the second son of James II and Blanche of Anjou. In 1314, aged 14, he married Teresa d'Entença y Cabrera, heiress of Urgell, who was the same age as him. Teresa's granduncle Ermengol X of Urgell had died childless in La Llitera; before his death, he had agreed to make Alfonso his heir, on condition that Alfonso would marry Teresa, who was his nearest kin. Alfonso was at this time only the second son of the king of Aragon. He and his father readily agreed to Ermengol's condition, and Alfonso married Teresa in 1314 in the Cathedral of Lerida. The teenage bridegroom is reputed to have been so liberal in the expenses during the wedding, that the local counsels imposed restrictions on how much he could spend. Alfonso and Teresa became the parents of seven children. Alfonso became heir to the throne in December 1319 after his older brother James renounced his rights to become a monk. During the reign of his father, Alfonso was the procurator-general of the Crown, and in 1323–1324, he undertook the conquest of Sardinia. Alfonso's father and first wife Teresa died within a few days of each other in 1327. Teresa died in childbirth on 20 October 1327, and James II died on 2 November 1327, whereupon Alfonso became king. In 1329, he began a long war with the Republic of Genoa. The city ofSassari had previously surrendered to Alfonso in 1323, but rebelled three more times; its possession was contested by Genoa, which led to the protracted war. In February 1329, Alfonso married Eleanor of Castile, the sister of king Alfonso XI of Castile. Eleanor had been briefly married to Alfonso's elder brother James the monk. That marriage, which James had refused to consummate, had been annulled in 1319–20. Eleanor had thereafter retired to a convent and had remained unmarried. By December the same year, the couple were rejoiced to become the parents of a son, Ferdinand, who was followed five years later by another son, John. Eleanor earnestly sought to advance the interests of her own infant sons over those of her stepson, the InfantePeter, who was the heir-apparent. She convinced her husband to grant very large and significant territories to her sons. on 28 December 1329, Alfonso granted his new-born son Ferdinand the Marquisate of Tortosa and the cities of Albarracín, Orihuela, Callosa d'en Sarrià, Guardamar, Alicante, Monforte, Elda, La Mola, Novelda and Aspe. Eleanor's younger son John, who was born five years later, was also granted several lordships when he was only a toddler: Elche, Biel and Bolsa were all bestowed upon him. These territories would be controlled by Eleanor, who had also received the city of Huesca and some other villages and castles belonging to the Aragonese crown at the time of her wedding. Nor was this all. While all of the above grants had been made from among the possessions of the Aragonese crown, the King also sought to bestow estates located within the Kingdom of Valencia upon the toddler Ferdinand, but he was prevented from doing so. When the King granted Ferdinand the cities of Xàtiva, Alzira, Sagunto, Morella, Burriana and Castellón de la Plana, all located in the Kingdom of Valencia, the local subjects protested, and for this reason the King decided to revoked these patents. These grants of land diminished the territorial patrimony of the crown and mainly affected the Infante Peter, Alfonso's son by his first wife, who however was too young to make any significant protest. However, the issue agitated the court, created a climate of resentment and divided the nobility into two camps. Alfonso died in January 1336, aged only 36. He was succeeded by Peter IV, his 16-year-old son from his first marriage.
