The Tongue River is a tributary of the Yellowstone River, approximately 265 mi (426 km) long, in the U.S. states of Wyoming and Montana. The Tongue rises in Wyoming in the Big Horn Mountains, flows generally northeast through northern Wyoming and southeastern Montana, and empties into the Yellowstone River at Miles City, Montana. Most of the course of the river is through the beautiful and varied landscapes of eastern Montana, including the Tongue River Canyon, the Tongue River breaks, the pine hills of southern Montana, and the buttes and grasslands that were formerly the home of vast migratory herds of American bison.

The Yellowstone River is a tributary of the Missouri River, approximately 692 miles (1,114 km) long, in the Western United States. Considered the principal tributary of upper Missouri, via its own tributaries it drains an area with headwaters across the mountains and high plains of southern Montana and northern Wyoming, and stretching east from the Rocky Mountains in the vicinity of Yellowstone National Park. It flows northeast to its confluence with the Missouri River on the North Dakota side of the border, about 25 miles west of present-day Williston.