The Stillwater River is a tributary of the Yellowstone River, approximately 70 km (113 km) long, in southern Montana in the United States.
It rises in the Absaroka-Beartooth Wilderness Area 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 Absaroka Range to the west and the Beartooth Mountains to the east, through Custer National Forest, past Nye and Absarokee. It joins the Yellowstone near Columbus.