The Gediz River (Turkish: Gediz Nehri, Turkish pronunciation: [ˈɟediz]) is the second-largest river in Anatolia flowing into the Aegean Sea. The ancient names Hermos and Hermus are sometimes still used.

The ancient name of the river was Hermos (Ἕρμος) or Hermus.

The name of the river Gediz may be related to the Lydian proper name Cadys; Gediz is also the name of a town near the river's sources. The name "Gediz" may also be encountered as a male name in Turkey.</ref>

The Hermos separated Aeolia from Ionia, except for Ionic Phocaea, which was north of the Hermos. The valley of the Hermos was the heartland of the ancient Lydian Empire and overlooking the valley was the Lydian capital Sardis.

In Turkey's Aegean Region, Gediz River's length is second only to Büyük Menderes River whose flow is roughly parallel at a distance of slightly more than a hundred kilometers to the south.

Gediz River rises from Murat Mountain and Şaphane Mountain in Kütahya Province and flows through Uşak, Manisa and İzmir Provinces. It joins the sea in the northern section of the Gulf of Izmir, close to the gulf's mouth, near the village of Maltepe in Menemen district, south of the coastal town of Foça.