Macedonian king of Egypt, son of Ptolemy II; he reunited Egypt and Cyrenaica and successfully waged the Third Syrian War (Syrian Wars) against the Seleucid kingdom.

Almost nothing is known of Ptolemy's youth before 245, when, following a long engagement, he married Berenice II, the daughter of Magas, king of Cyrene; thereby he reunited Egypt and Cyrenaica, which had been divided since 258. Shortly after his accession and marriage, Ptolemy invaded Coele Syria, to avenge the murder of his sister, the widow of the Seleucid king Antiochus II. Ptolemy's navy, perhaps aided by rebels in the cities, advanced against Seleucus II's (Seleucus II Callinicus) forces as far as Thrace, across the Hellespont, and also captured some islands off the Asia Minor coast, but were checked c. 245. Meanwhile, Ptolemy, with the army, penetrated deep into Mesopotamia, reaching at least Seleucia on the Tigris, near Babylon. According to classical sources he was compelled to halt his advance because of domestic troubles. Famine and a low Nile, as well as the hostile alliance between Macedonia, Seleucid Syria, and Rhodes, were perhaps additional reasons. The war in Asia Minor and the Aegean intensified as the Achaean League, one of the Greek confederations, allied itself to Egypt, while Seleucus II secured two allies in the Black Sea region. Ptolemy was pushed out of Mesopotamia and part of North Syria in 242–241, and the next year peace was finally achieved. Ptolemy managed to keep the Orontes River region and Antioch, both in Syria; Ephesus, in Asia Minor; and Thrace and perhaps also Cilicia.

Within Egypt, Ptolemy continued the colonization of al-Fayyūm (the oasis-like depression southwest of Cairo), which his father had developed. He also reformed the calendar, adopting 311 as the first year of a “Ptolemaic Era.” The Canopus decree, a declaration published by a synod of Egyptian priests, suggests that the true duration of the year (365 ¹/₄ days) was now recognized, for an extra day was added to the calendar every four years. The new calendar failed, however, to achieve popular acceptance. The priests and classical sources also credited Ptolemy with the restoration of the divine statues plundered from the temples during Persian rule. In addition, the King initiated construction at Edfu, the Upper Egyptian site of a great Ptolemaic temple, and made donations to other temples.

Ptolemy avoided involvement in the wars that continued to plague Syria and Macedonia. He did, however, send aid to Rhodes after earthquakes devastated the island, but he refrained from subsidizing the schemes of the Spartan king against Macedonia, though he granted him asylum in 222. In Asia Minor, when a pretender to one of the kingdoms, who was the instigator of much of the trouble there, sought asylum in Ptolemaic territory, Ptolemy promptly interned him. His policy was to maintain an equilibrium of power, guaranteeing the safety of his own territory. After declaring his son his successor, Ptolemy died, leaving Egypt at the peak of its political power and internally stable and prosperous.