Ptolemy IX (Ptolemy Lathyrus) (tŏl´əmē ləthī´rəs), d. 81 BC, king of ancient Egypt (116–107 BC, 88–81 BC) of the Macedonian dynasty, son of Ptolemy VIII and the younger Cleopatra. He is also called Ptolemy Soter II. His mother ruled jointly with him and held the actual power. She forced him to accept (110 BC) as coruler his brother, Ptolemy Alexander (Ptolemy X), who drove him from the throne in 107 BC Ptolemy IX went to Cyprus and then to Syria. He returned to conquer Cyprus, and in 88 BC he reconquered Egypt.