(Ἀχαΐος), son of Andromachus, whose sister Laodice married Seleucus Callinicus, the father of Antiochus the Great. Achaeus himself married Laodice, the daughter of Mithridates, king of Pontus. (Plb. 4.51.4, 8.22.11.) He accompanied Seleucus Ceraunus, the son of Callinicus, in his expedition across mount Taurus against Attalus, 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. Antiochus the Great, the successor of Seleucus, appointed him to the command of all Asia on this side of mount Taurus, B. C. 223. Achaeus recovered for the Syrian empire all the districts which Attalus had gained; but having been falsely accused by Hermeias, the minister of Antiochus, of intending to revolt, he did so in self-defence, assumed the title of king, and ruled over the whole of Asia on this side of the Taurus. As long as Antiochus was engaged in the war with Ptolemy, he could not march against Achaeus; but after a peace had been concluded with Ptolemy, he crossed the Taurus, united his forces with Attalus, deprived Achaeus in one campaign of all his dominions and took Sardis with the exception of the citadel. Achaeus after sustaining a siege of two years in the citadel at last fell into the hands of Antiochus B. C. 214, through the treachery of Bolis, who had been employed by Sosibius, the minister of Ptolemy, to deliver him from his danger, but betrayed him to Antiochus, who ordered him to be put to death immediately. (Plb. 4.2.6, 4.48, 5.40.7, 42, 57, 7.15_18, 8.17-23.)