Approximately 70 miles (113 km) long,[3] it runs through southern Montana in the United States.

[4] The Stillwater River rises at the north slope of Miller Mountain,[1] south of the Beartooth Mountains in southern Park County, near the state line with Wyoming and the boundary of Yellowstone National Park.

It flows northeast, between the Absaroka Range to the west and the Beartooth Mountains to the east, through Custer National Forest, past Nye and Absarokee.

In the spring Rainbow Trout from the Yellowstone River enter the Stillwater to spawn, some traveling as far upstream as Nye.

Despite its misleading name, the Stillwater's most popular stretches contain class II and III whitewater.