Peter III of Aragon (Catalan: Pere, Spanish: Pedro) (1239 – November 2, 1285, also Peter I of Valencia, Peter II of Barcelona), known as the Great, was the king of Aragon and Valencia and count of Barcelona from 1276 to 1285.

In 1262, he married Constance, daughter and heiress of Manfred of Sicily. He became also King Peter I of Sicily from 1282 after the Sicilian Vespers expulsed the French from the island. His navy under the admiral Roger of Lauria defeated the invading French crusade at Roses, and his armies crushed the retreating French crusaders at the Coll de Panissars in 1285.

Peter himself was the direct descendant and the heir-general of Mafalda of Hauteville, daughter of Robert Guiscard, Duke of Apulia, the Norman conqueror, and his official wife Sigelgaita of Salerno, a Langobard princess. After Dukes of Apulia became extinct with William II in 1128, then apparently Mafalda's heirs (then counts of Barcelona) became de jure heirs of the Guiscard and Sigelgaita, and thus Peter was dormantly a claimant to the Norman succession of Southern Italy. Two Sicilies were to be a most pursued inheritance for the Aragonese Royal House and its heirs for the next five centuries.

Peter left Aragon to his eldest son Alfonso III of Aragon, and Sicily to his second son James I of Sicily. Peter's third son, Infante don Fadrique, later after his brother James, became regent of Sicily and then its king, too. Peter had also a daughter, Elizabeth (Elisabet), who married Denis of Portugal.

In the Divine Comedy Dante sees Peter "singing in concert" with his former rival Charles I of Sicily outside the gates of Purgatory.