Peter III (ca. 1239-1285) was king of Aragon and count of Barcelona from 1276 to 1285 and king of Sicily from 1282 to 1285. He was one of medieval Spain's greatest rulers.

The son of King James I of Aragon and Violante (Yolanda) of Hungary, Peter (or Pedro) III inherited the crown of Aragon in 1276, after his father's extensive conquests had increased both Aragonese power and prestige. In 1262 Peter had married Constance, daughter of Manfred of Sicily and granddaughter of the emperor Frederick II, and had inherited in 1266 the Hohenstaufen family claim to the kingdom of Sicily. Aragon's geographical and economic orientation toward the Mediterranean, Aragonese claims on the kingdom of Sicily, and Peter's great personal abilities made him the first monarch in the Spanish peninsula to participate actively and successfully in the wider affairs of Europe and the Mediterranean.

At the beginning of his reign Peter committed his resources to the...