Ptolemy IX (Ptolemy Lathyrus)tŏlˈəmē ləthĪˈrəs, d. 81 B.C., king of ancient Egypt (116–107 B.C., 88–81 B.C.) 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 B.C.) as coruler his brother, Ptolemy Alexander (Ptolemy X), who drove him from the throne in 107 B.C. Ptolemy IX went to Cyprus and then to Syria. He returned to conquer Cyprus, and in 88 B.C. he reconquered Egypt.