The Stillwater River is a 69.3 mi tributary of the Great Miami River in western Ohio in the United States. Via the Great Miami and Ohio rivers, it is part of the Mississippi River watershed. It rises near the Indiana state line, in western Darke County, approximately 10 mi northwest of Greenville. It flows east-southeast and is joined by Greenville Creek in Covington, approximately 5 mi west of Piqua. It flows south past Covington and Englewood, where it is dammed for flood control, then southeast to join the Great Miami River in Dayton. Stillwater River was so named on account of its relatively slow current. The Stillwater River was one of the Great Miami River tributaries that flooded during the Great Dayton Flood of 1913, resulting in the creation of the Miami Conservancy District.

Variant names

According to the Geographic Names Information System, the Stillwater River has also been known as: