Achaeus of Eretria (Greek: Aχαιός; born 484 BC in Euboea) was a Greek playwright author of tragedies and satyr plays, variously said to have written 24, 30, or 44 plays, of which 19 titles are known, some of which include Adrastus, Linus, Cycnus, Eumenides, Philoctetes, Pirithous, Theseus, and Œdipus.
His first play was produced in 447 BC and won a prize. A quote in Aristophanes' The Frogs suggests he was dead by 405 BC. Some classicists suggest that the fact that he only won a single prize was due to his non-Athenian birth, as the men of Athens were loath to honor any but their own fellow-citizens.
Achaeus of Eretria belongs to the classic age, but is not recognized as a classic writer. His satyric plays were much admired for their spirited style, albeit somewhat labored and lacking in clearness. The philosopher Menedemus thought his plays second only to Aeschylus, he was part of the Alexandrian canon, and Didymus wrote a commentary on him. Athenaeus (10.451c) describes him as having a lucid style, but with tendencies to obscurity. Athenaeus also claimed that Euripides took a line from Achaeus, while Aristophanes quotes him twice, in The Frogs and The Wasps.