The Jefferson River is a tributary of the Missouri River, approximately 207 miles (333 km) long, in the U.S. state of Montana. It is one of three rivers, along with the Madison and Gallatin that converge near Three Forks, Montana to form the Missouri. Along various stretches of its lenghth, it is also known as the Red Rock River and the Beaverhead River.

It rises as the Red Rock River in the Centennial Mountains near the continental divide in southwestern Beaverhead County, near Montana's border with Idaho, and flows west through Upper and Lower Red Rock lakes, then NNW past Lima to the Clark Canyon Reservoir, where it becomes the Beaverhead. As the Beaverhead, it flows NNE past Dillon. Near Twin Bridges, it is joined by the Ruby River and the Big Hole River, and continues as the Jefferson River north and east. Near Cardwell, it receives the Boulder River, and flows east to form the Missouri with the Madison and Gallatin northeast of Three Forks, approximately 28 mi (45 km) WNW of Bozeman.

The Jefferson was named for U.S. President Thomas Jefferson by Lewis and Clark in 1805; the expedition traveled from Three Forks up the Jefferson and Beaverhead to a point presently beneath the waters of Clark Canyon Reservoir, where they left the Beaverhead to travel west up Horse Prairie Creek.

See also

• List of Montana rivers

Rivers of Montana | Tributaries of the Missouri River

Jefferson River (Montana) | Rio Jefferson