Ptolemy IX Soter II or Lathyros ("grass pea") (Greek: Πτολεμαῖος Σωτήρ Λάθυρος, Ptolemaĩos Sōtḗr Láthuros) was king of Egypt three times, from 116 BC to 110 BC, 109 BC to 107 BC and 88 BC to 81 BC, with intervening periods ruled by his brother, Ptolemy X Alexander.

At first he was chosen by his mother Cleopatra III to be her co-regent (his father Ptolemy VIII wished that she would rule with one of her sons), though she was more forced to choose him by the Alexandrians. He married his sister Cleopatra IV, but his mother pushed her out and replaced her with his younger sister Cleopatra Selene. Later, she claimed that he tried to kill her, and successfully deposed him, putting her favorite son Alexander on the throne as co-regent with her. However, she later grew tired of the now Ptolemy X and deposed him, putting Ptolemy IX back on the throne. She was soon murdered by Ptolemy X, who took the throne again. He was then killed in battle, and Ptolemy IX reigned until his own death.

His daughter Berenice III took the throne after his death, and reigned for about a year. She was forced to marry her stepson Alexander, who reigned under the name Ptolemy XI Alexander II and had her killed nineteen days later.

He was shortly lynched by the enraged Alexandrian Mob - to stave off invasion and/ or annexation by other powers, those with influence ensured the throne passed to Lathyros' remaining children, though they were illegitimate: Ptolemy XII Auletes and his brother or half-brother (also named Ptolemy) were recalled from the Kingdom of Pontus; the elder was placed on the throne and given a queen- in the form of his sister/ half-sister, Cleopatra V- and the younger Ptolemy was given rule of the last external territory Egypt possessed, the isle of Cyprus.

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