The Achaeans (Greek: Ἀχαιοί, Achaioi, Αχαιοί) were one of the four major tribes into which the people of Classical Greece divided themselves. According to the foundation myth formalized by Hesiod, their name comes from Achaeus, the mythical founder of the Achaean tribe, who was supposedly one of the sons of Xuthus, and brother of Ion, the founder of the Ionian tribe. Xuthus was in turn the son of Hellen, the mythical patriarch of the Greek (Hellenic) nation.

Historically, the members of the Achaean tribe inhabited the region of Achaea in the northern Peloponnese. Unlike the other major tribes (Ionians, Dorians and Aeolians), the Achaeans did not have a separate dialect in the Classical period, instead using a form of Doric.

The origin of the Achaeans is somewhat problematic. Homer, probably writing in the 9th century BC, uses the term Achaeans as a generic term for Greeks throughout the Iliad, which is believed to describe events in Mycenaean Greece, i.e. before 1200 BC. However, there is no firm evidence that the Greeks of the Mycenaean period used that name to describe themselves. The term Ahhiyawa, found in 13th century BC Hittite texts may mean "Achaeans", that is to say the Greeks of the Mycenean culture, but again, there is no definitive evidence that this is the case. Emil Forrer went as far to claim that there existed a "great empire" called Ahhiyawa, which stood as equal by the side of the old states of the east. However, his conclusions were disproven by later researchers, especially by Ferdinand Sommer.