The Bighorn River is a tributary of the Yellowstone, approximately 461 miles (742 km) long, in the western United States in the states of Wyoming and Montana.
The river was named in 1805 by fur trader François Larocque for the Bighorn Sheep he saw along its banks as he explored the Yellowstone River. The upper reaches of the Bighorn, south of the Owl Creek Mountains in Wyoming, are known as the Wind River. The two rivers are sometimes referred to as the Wind/Bighorn. The Wind River officially becomes the Bighorn River at the Wedding of the Waters, on the north side of the Wind River Canyon near the town of Thermopolis. From there, it flows through the Bighorn Basin in North Central Wyoming, passing through Thermopolis and Hot Springs State Park. At the border with Montana, the river turns northeast, and flows past the north end of the Bighorns, through the Crow Indian Reservation, where the Yellowtail Dam forms the reservoir Bighorn Lake. The reservoir and the surrounding gorge are part of the Bighorn Canyon National Recreation Area. It is joined by the Little Bighorn River near the town of Hardin, Montana. Approximately fifty miles farther downriver, it joins the Yellowstone.