| Saint Marys River | |
| Map: | Maumeerivermap.png |
| Subdivision Type1: | Country |
| Subdivision Name1: | United States |
| Subdivision Type2: | States |
| Source1: | confluence of East Branch and Center Branch |
| Source1 Coordinates: | 40.5372°N -84.3628°W |
| Mouth Coordinates: | 41.0833°N -85.1322°W |
| Progression: | St. Marys River → Maumee River → Lake Erie → Great Lakes → St Lawrence River → Gulf of St. Lawrence |
| Source1 Elevation: | 854feet |
| Mouth Elevation: | 745feet |
| Length: | 99miles |
| Basin Size: | 793.1sqmi[1] |
| Custom Label: | GNIS ID |
The St. Marys River (Shawnee: Kokothikithiipi,[2] in Miami-Illinois: Nameewa siipiiwi)[3] is a 99adj=midNaNadj=mid[4] tributary of the Maumee River (Miami-Illinois: Taawaawa siipiiwi) in northwestern Ohio and northeastern Indiana in the United States. Prior to development, it was part of the Great Black Swamp. Today, 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 west-northwest, flowing past Rockford and Willshire into Adams County, Indiana. 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.
The World War II-era US Navy vessel St. Mary's River was named after this river.