Ptolemy III (Ptolemy Euergetes)tŏlˈəmē yōûrˈjĭtēz, d. 221 B.C., king of ancient Egypt (246–221 B.C.), of the Macedonian dynasty, son of Ptolemy II and the first Arsinoë. He plunged immediately into a war with Syria, where his sister, Berenice, was trying to secure the throne for her son. Berenice and her son were murdered before Ptolemy could arrive, and Seleucus II held the throne, though the Egyptian king won a brilliant if impermanent victory. Egyptian fleets controlled most of the coasts of Asia Minor and E Greece, and the kingdom was enlarged by Ptolemy's marriage to Berenice, daughter and heiress of the king of Cyrene.