The Ontonagon River (/ˌɒntəˈnɑːɡən/ ON-tə-NAH-gən) is a river flowing into Lake Superior at the village of Ontonagon, on the western Upper Peninsula of Michigan in the United States.

The main stem of the river is 25 miles (40 km) long[4] and is formed by a confluence of several longer branches, portions of which have been collectively designated as a National Wild and Scenic River.

[6] On March 3, 1992, the following reaches of the Ontonagon's upper tributaries were collectively designated the Ontonagon National Wild and Scenic River: The upper courses of the East and Middle branches in the Ottawa National Forest; the Cisco Branch in its entirety; and approximately the middle section of the West Branch, from Cascade Falls to the Victoria Reservoir.

[5][6] A very large mass of solid, nearly pure copper, the Ontonagon Boulder, was removed from the west branch of the Ontonagon River and transported in 1842 to Detroit, where it was weighed at 3708 pounds.

[13] It is displayed in Washington, D.C., at the Smithsonian National Museum of Natural History.