Peter was the eldest son of James I of Aragon and his second wife Violant of Hungary. Among opportunistic betrothals of his youth, he was betrothed to Eudoxia Laskarina, the youngest daughter of Theodoros II Laskaris, in or before 1260. This contract was dissolved, however, after Eudoxia's brother lost the imperial throne in 1261, and Eudoxia was instead married to the Count of Tenda. On 13 June 1262, Peter married Constance II of Sicily, daughter and heiress of Manfred of Sicily. During his youth and early adulthood, Peter gained a great deal of military experience in his father's wars of the "Reconquista" against the Moors.