In Greek mythology, Achaeus was the son of Xuthus and Creusa (by the way, this is the Roman spelling of the name, while the Greek spelling is Achaios). He also was the brother of Ion and, according to some versions, of Dorus.
On his father's side, he was a grandson of Hellen (which, in turn, was the son of Deucalion) and nephew of Aeolus, and on his mother side, he was a grandson of Erechtheus, the king of Athens.
His father, Xuthus, was exiled from Thessaly and came to Aegialus. After his death, Achaeus gathered troops from Aegialus and Athens and managed to reconquer the southern region of Thessaly, which was later called Achaea after him. Of course, the inhabitants of this region were called Achaeans.
Achaeus had two sons, Architeles and Archander, who travelled to Peloponnesus and so the northern part of this region was called Achaea, too.
In the Illiad, Homer uses the name Achaeans as a generic name for all the Greeks who fought in the war against Troy.
In Euripides' play Ion, the facts are mainly the author's invention. He makes Ion Apollo's son, and so the Ionians are superior to the Achaeans and the Dorians, because their forefather is a god.
In real life, there were other persons with the same name:
- Achaeus of Eretria was a playwright born in the year 484 BC. 19 of the titles of his plays are still known. He won a prize in 447 BC.
- The nobleman Achaeus (also called "the Elder"), who was the second son of the king Seleucus I Nicator. The Seleucids fought against the Galatians and some of their supporters were taken prisoners. The nobleman paid the whole ransom for them and, in order to show their gratitude, the descendants of the people freed by him would sacrifice an ox every year in his memory, in the Sanctuary of Zeus.
- The general Achaeus, son of Andromachus and grandson of the nobleman mentioned above.