The Belle Fourche River (pronounced bel FOOSH; ) is a tributary of the Cheyenne River, about long, in the U.S. states of Wyoming and South Dakota. It is part of the Mississippi River watershed via the Cheyenne and Missouri Rivers. In the latter part of the 19th century, the Belle Fourche River was known as the North Fork of the Cheyenne River. Belle Fourche is a name derived from French, meaning "beautiful fork".
It rises in northeastern Wyoming, in southern Campbell County, around southwest of Wright. It flows northeast around the north side of the Bear Lodge Mountains, past Moorcroft and Devils Tower. Near the state line with Montana, it turns abruptly southeast and flows in western South Dakota, past Belle Fourche and around the north side of the Black Hills. In southern Meade County near Hereford, it turns east-northeast and joins the Cheyenne roughly east-northeast of Rapid City.
The point at which the river flows out of Wyoming and into South Dakota is the lowest elevation point in Wyoming at . This is the second-highest low point of any U.S. state. At Elm Springs, the river has an average discharge of
The river provides significant recreation and irrigation for agriculture in western South Dakota. The total irrigation area of the river in South Dakota is approximately .