Achaeus may refer to:
- Achaeus (son of Xuthus), mythical founder of Achaean race
- Achaeus of Eretria (born 484 BC), tragic poet who wrote forty-five tragedies, some of whose titles are preserved
- Achaeus of Syracuse, another tragic poet who wrote ten or fourteen tragedies
- Achaeus (son of Seleucus I Nicator), son of Seleucus I Nicator and brother of Antiochus I Soter
- Achaeus (general) (died 213 BC), grandson to the above and a relative of Antiochus III the Great who was appointed governor of all the king's provinces beyond Taurus Mountains
- Achaeus (genus), a genus of crabs