The St. Marys River is a tributary of the Maumee River, approximately 100 mi (161 km) long, in western Ohio and eastern Indiana in the United States. It drains a primarily rural farming region in the watershed of Lake Erie.

It is formed in southern Auglaize County in western Ohio by the confluence of the short East Branch and Center Branch. It flows briefly west to St. Marys, approaching to within two miles of Grand Lake before turning to the north. In northwestern Auglaize County it turns sharply to the WNW, flowing past Rockford and Willshire into Adams County, Indiana. In northwestern Indiana it flows northwest past Decatur, then enters Fort Wayne. It hooks around in its last half mile (0.25 km) join the St. Joseph River from the west to form the Maumee in downtown Fort Wayne.

See also

• List of Indiana rivers
• List of Ohio rivers