Achaemenid king of Persia (reigned 359/358–338 BC).

He was the son and successor of Artaxerxes II and was called Ochus before he took the throne. Artaxerxes III was a cruel but energetic ruler. To secure his throne he put to death most of his relatives. In 356 he ordered all the satraps (governors) of the Achaemenid empire to dismiss their mercenaries. He also forced Athens to conclude peace and to acknowledge the independence of its rebellious allies (355).

Artaxerxes then attempted to subjugate Egypt (Egypt, ancient), which had been independent since 404. Failure of the first attempt (351) encouraged the Phoenician towns and the princes of Cyprus to revolt. Early in 345, Artaxerxes collected a great army and marched against the Phoenician city of Sidon. Mentor of Rhodes, who had helped betray Sidon, rose high in the king's favour and entered into a close understanding with the eunuch Bagoas, the king's favourite. Artaxerxes then advanced on Egypt with a great land and naval force and, at Pelusium in the Nile River delta, defeated the pharaoh Nectanebo II (343). A Persian satrap was placed over Egypt, the walls of its cities were destroyed, its temples were plundered, and Artaxerxes was said to have killed the Apis bull with his own hand.

After the king's return to Susa, Bagoas ruled the court and the upper satrapies, while Mentor restored the authority of the empire throughout the west. When Philip of Macedon (Philip II) attacked Perinthus and Byzantium (340), Artaxerxes sent support to those cities. In 338 Artaxerxes and his elder sons were killed by Bagoas, who then raised the king's youngest son, Arses, to the throne.