Ptolemy IX Soter II, commonly nicknamed Lathyros, was twice King of Ptolemaic Egypt. He was the son of Ptolemy VIII and Cleopatra III. He reigned as Ptolemy Philometor Soter in joint rule with his grandmother Cleopatra II and mother Cleopatra III from 116 to 107 BC, and then again as Ptolemy Soter from 88 to 81 BC.

The Ptolemaic Kingdom or Ptolemaic Empire was an Ancient Greek polity based in Egypt during the Hellenistic period. It was founded in 305 BC by the Macedonian general Ptolemy I Soter, a companion of Alexander the Great, and ruled by the Ptolemaic dynasty until the death of Cleopatra VII in 30 BC. Reigning for nearly three centuries, the Ptolemies were the longest and final dynasty of ancient Egypt, heralding a distinctly new era for religious and cultural syncretism between Greek and Egyptian culture.