The St. Joseph River is an 86.1-mile-long (138.6 km) tributary of the Maumee River in northwestern Ohio and northeastern Indiana in the United States, with headwater tributaries rising in southern Michigan. It drains a primarily rural farming region in the watershed of Lake Erie.

The Maumee River is a river running in the United States Midwest from northeastern Indiana into northwestern Ohio and Lake Erie. It is formed at the confluence of the St. Joseph and St. Marys rivers, where Fort Wayne, Indiana has developed, and meanders northeastwardly for 137 miles (220 km) through an agricultural region of glacial moraines before flowing into the Maumee Bay of Lake Erie. The city of Toledo is located at the mouth of the Maumee. The Maumee was designated an Ohio State Scenic River on July 18, 1974. The Maumee watershed is Ohio's breadbasket; it is two-thirds farmland, mostly corn and soybeans. It is the largest watershed of any of the rivers feeding the Great Lakes, and supplies five percent of Lake Erie's water.