The Achelous, also Acheloos, is a river in Epirus, western Greece. It is 220 km (137 mi) long. It formed the boundary between Acarnania and Aetolia of antiquity. It empties into the Ionian Sea. In ancient times its spirit was venerated as the river god Achelous.

Acarnania is a region of west-central Greece that lies along the Ionian Sea, west of Aetolia, with the Achelous River for a boundary, and north of the gulf of Calydon, which is the entrance to the Gulf of Corinth. Today it forms the western part of the regional unit of Aetolia-Acarnania. The capital and principal city in ancient times was Stratos. The north side of Acarnania of the Corinthian Gulf was considered part of the region of Epirus.