Download coordinates as: The St. Marys River (Shawnee: Kokothikithiipi,[2] in Miami-Illinois: Nameewa siipiiwi)[3] is a 99-mile-long (159 km)[4] tributary of the Maumee River (Miami-Illinois: Taawaawa siipiiwi) in northwestern Ohio and northeastern Indiana in the United States.

Today, it drains a primarily rural farming region in the watershed of Lake Erie.

It flows briefly west to St. Marys, approaching to within two miles of Grand Lake before turning to the north.

In northeastern Indiana it flows northwest through Decatur, then enters Fort Wayne.

It hooks around in its last half mile (0.8 km) to join the St. Joseph River (in Miami-Illinois: Kociihsasiipi) from the west to form the Maumee in downtown Fort Wayne.