Achaeus (Ancient Greek: Ἀχαιός; flourished 3rd century BC) was a Seleucid nobleman and was the second son born to King and founder of the Seleucid Empire Seleucus I Nicator and Sogdian noblewoman Apama I.
He had three siblings: one brother the Seleucid King Antiochus I Soter and two sisters: Apama and Laodice.
The Seleucid military campaign against the Galatians took place between 269-267 BC, during the reign of Antiochus I.
Those who had Achaeus as their benefactor inscribed their benefaction on a stone stele and placed it in the sanctuary of Zeus at Babakome and that of Apollo at Kiddioukome.
The descendants of those who were saved by Achaeus for all time were granted a seat of honor at the public festivals and sacrificed an ox to Achaeus every year in the sanctuary of Zeus.