The Ionians (/aɪˈoʊniənz/; Greek: Ἴωνες, Íōnes, singular Ἴων, Íōn) were one of the four major tribes that the Greeks considered themselves divided into, during the ancient period; alongside Dorians, Aeolians and Achaeans. The Ionian dialect was one of the three major linguistic divisions of the Hellenic world, together with the Dorian and Aeolian dialects.

When referring to populations, "Ionian" defines several groups in Classical Greece. In the narrowest sense, it referred to the region of Ionia in Asia Minor. In a broader sense, it could be used to describe all speakers of the Ionic dialect, which in addition to those in Ionia proper also included the populations of Euboea, the Cyclades and many colonies founded by Ionian colonists. Finally, in the broadest sense, it could be used to describe all those who spoke languages of the East Greek group, which included Attic.

The Foundation myth which was current in the Classical period suggested that the Ionians were named after Ion, son of Xuthus, who lived in the north Peloponnesian region of Aegilaus. When the Dorians invaded the Peloponnese and expelled the Achaeans from the Argolid and Lacedaemonia, the Achaeans moved into Aegilaus (henceforth known as Achaea) and the Ionians were in turn expelled. The Ionians moved to Attica and mingled with the local population. Later, many people emigrated to the coast of Asia Minor founding the historical region of Ionia.