Mysia was a region in the northwest of Asia Minor. It was located on the south coast of the Sea of Marmara. It was bounded by Lydia and Phrygia on the south, by Bithynia on the northeast, and by the Propontis and Aegean Sea on the north and west. In ancient times it was inhabited by the Mysi, by Aeolian Greeks, and by other groups.

The precise limits of Mysia are difficult to assign. The Phrygian frontier was fluctuating, while in the northwest the Troad was only sometimes included in Mysia. The northern portion was known as Mysia Minor or Hellespontica, while the southern was called Major or Pergamene.

The chief physical features of Mysia are the two mountains—Mount Olympus at (7600 feet) in the north and Mount Temnus in the south, which for some distance separates Mysia from Lydia and is afterwards prolonged through Mysia to the neighbourhood of the Gulf of Adramyttium . The major rivers in the northern part of the province are the Macestus and its tributary, the Rhyndacus , both of which rise in Phrygia, and, after diverging widely through Mysia, unite their waters below the lake of Apollonia about 15 miles from the Propontis. The Caïcus in the south rises in Temnus, and from thence flows westward to the Aegean Sea, passing within a few miles of Pergamon. In the northern portion of the province are two considerable lakes, Artynia , or Apolloniatis (Abulliont Geul), and Aphnitis (Maniyas Geul), which discharge their waters into the Macestus from the east and west respectively.

The most important cities were Pergamon in the valley of the Caïcus, and Cyzicus on the Propontis. The whole sea-coast was studded with Greek towns, several of which were places of considerable importance; thus the northern portion included Parium , Lampsacus and Abydos, and the southern Assus, Adramyttium. Further south, on the Eleatic Gulf , were Elaea , Myrina and Cyme .

A minor episode in the Trojan War cycle in Greek mythology has the Greek fleet land at Mysia, mistaking it for Troy. Achilles wounds their king, Telephus, after he slays a Greek; Telephus later pleads with Achilles to heal the wound.

See also: Aeolis.