The name of the river may come from the Blackfoot word mokowan of mokoan, meaning 'stomach'.

[2] The Belly River originates in northwestern Montana at Helen Lake, near the base of Ahern Peak in Glacier National Park.

It is crossed by Highway 2 north of Stand Off, then continues north along the base of the Mokowan Ridge, where it receives the waters of the Waterton River.

Its flow becomes meandered before the river turns east around the Wild Turnip Hill, then it empties into the Oldman River west of Coalhurst, south of Highway 3, at an elevation of 875 meters (2,871 ft).

From spring to mouth, the Belly River receives the following tributaries: