Ptolemy V (Ptolemy Epiphanes) (tŏl´əmē ĬpĬf´ənēz), d. 180 BC, king of ancient Egypt (205–180 BC), of the Macedonian dynasty, son of Ptolemy IV. He succeeded to the throne as a small boy, and his reign began with disastrous civil wars. Invasions by Antiochus III of Syria and Philip V of Macedon cost Egypt all of Palestine and the Egyptian possessions in Asia Minor. Antiochus defeated Ptolemy decisively at the Battle of Panion in 200 BC Peace was confirmed by the marriage of Ptolemy to Cleopatra, daughter of Antiochus. Egypt was much weakened when his reign ended. The Rosetta Stone (see under Rosetta) inscriptions concern Ptolemy's reign and his relations with the priests.