The Mysi (Mysians) were the inhabitants of Mysia, a region in northwest Asia Minor. Herodotus wrote that they were brethren of the Carians and Lydians (Herod. 1.171), and that the Mysians were "Lydian colonists" (Herod. 7.74). This identification may be supported by the fact that only Mysians, Carians, and Lydians were allowed to worship at the temple of Carian Zeus in the country of the Mylasians (Herod. 1.171), based on the tradition that the eponymous figures Car (Carians), Lydus (Lydians), and Mysus (Mysians) were brothers (Herod. 1.171).

The affinity of the Mysian language or dialect, while almost certainly Indo-European, is not well understood, but may have been akin to Carian and Lydian, or to Phrygian. There is also an ancient reference on the affinity of the Mysian tongue to the Paionian language of Paionia in Europe.

There are inscriptions discovered in Mysia which may represent the Mysian language or dialect, including:

Likis: braterais patrizi isk.

It shows typical Indo-European characteristics.

The Mysians fought in the Trojan War on the side of Troy. Herodotus recorded the tradition that Mysians (along with Teucrians) invaded Europe, conquering "all of Thrace" and invading Greece as far as Elis in early times (7.20).