ancient district in northwest Anatolia adjoining the Sea of Marmara on the north and the Aegean Sea on the west. A vague inland perimeter was bounded by the districts of Lydia on the south and Phrygia and Bithynia on the east. Mysia designated a geographic rather than a political territory and encompassed Aeolis, Troas, and the region surrounding Pergamum.

Homer mentioned the Mysians (for whom the region was named) as primitive allies of the Trojans, but historically there is no record of their action as an independent nation. Mysia was ruled successively by Lydia, Persia, and Pergamum, after which it was incorporated into the Roman province of Asia (129 BC).