The Rapid River is a river in Algoma District, Ontario, Canada. The river is about long and begins at Seymour Lake at an elevation of . It travels west and takes in its left tributary the Little Rapid River at an elevation of . The river continues west over a small rapids and a waterfall before emptying into the Mississagi River at an elevation of . A road, which begins at Highway 129 north of the river's mouth, parallels the majority of the river to Seymour Lake.