Achaeus (Ancient Greek: Ἀχαιός; died 213 BC) was a general and later a separatist ruler of part of the Greek Seleucid kingdom. He was the son of Andromachus, whose sister Laodice II married Seleucus Callinicus, the father of Antiochus III the Great. Achaeus himself married Laodice of Pontus, one of the daughters to Laodice and Mithridates II of Pontus. He accompanied Seleucus Ceraunus, the son of Callinicus, in his expedition across mount Taurus against Attalus I, and after the assassination of Seleucus revenged his death; and though he might easily have assumed the royal power, he remained faithful to the family of Seleucus.
In 223 BC Antiochus III, the successor of Seleucus, appointed him to the command of all Asia Minor on the western side of Mount Taurus. Achaeus recovered all the districts which Attalus had gained for the Seleucids once more; but being falsely accused by Hermeias, the minister to Antiochus, of intending to revolt, he did so in self-defence, assumed the title of king, and ruled over the whole of Asia on the western side of the Taurus. As long as Antiochus was engaged in the war with Ptolemy, he would not march against Achaeus; but upon the conclusion of a treaty with Ptolemy, he crossed the Taurus, uniting his forces with Attalus, and in one campaign deprived Achaeus of his dominions and took Sardis (with the exception of the citadel). After sustaining a siege of two years, the citadel at last fell into the hands of Antiochus in 213 BC, through the treachery of Bolis (who had been employed by Sosibius, minister to Ptolemy). Bolis pledged to deliver Achaeus to safety, but turned him over to Antiochus, who immediately put him to death.
- Bement, R. B.; The kingdom of Brass (1856)
- Polybius; Histories, Evelyn S. Shuckburgh (translator); London - New York, (1889)
- Smith, William (editor); Dictionary of Greek and Roman Biography and Mythology, "Achaeus (2)", Boston, (1867)
- This article incorporates text from a publication now in the public domain: Smith, William, ed. (1870). "article name needed". Dictionary of Greek and Roman Biography and Mythology.