Antiochus IX Eusebes Cyzicenus (, "Antiochus the Pious, the Cyzicene") was a ruler of the Hellenistic Seleucid kingdom. He was the son of Antiochus VII and Cleopatra Thea. He left the kingdom in 129 BC and went to the city of Cyzicus, but he returned in 116 BC to challenge his half-brother Antiochus VIII for power.
The siblings fought a twenty-year civil war. In 112 BC, Antiochus IX's wife, Cleopatra IV, was killed by her sister Tryphaena, the wife of Antiochus VIII. Tryphaena herself died shortly afterwards. Antiochus VIII was assassinated in 96 BC; he was succeeded by his sons Seleucus VI and Demetrius III. Antiochus IX then took the capital Antioch and married his deceased wife's sister Cleopatra Selene, who was herself the widow of Antiochus VIII. Seleucus VI continued the war against his uncle. Antiochus IX Eusebes Cyzicenus was killed in battle in 95 BC.
Biography
thumb|Bust probably depicting Antiochus IX.
The son of Antiochus VII and Cleopatra Thea, by her sister and rival Tryphaena, wife of Antiochus VIII. Antiochus VIII was assassinated in 96 BC. Antiochus IX then captured Antioch and married his brother's widow, Cleopatra Selene, who was the sister of Cleopatra IV.