Ontonagon County is a county in the Upper Peninsula of the U.S. state of Michigan. As of the 2020 census, the population was 5,816, making it Michigan's third-least populous county. The county seat is Ontonagon. The county was set off in 1843, and organized in 1848. Its territory had been organized as part of Chippewa and Mackinac counties. With increasing population in the area, more counties were organized. After Ontonagon was organized, it was split to create Gogebic County. It is also the westernmost county in United States that lies within the Eastern Time Zone.

Ontonagon is a village in the Upper Peninsula of the U.S. state of Michigan. The population was 1,285 at the 2020 census. It is the county seat of Ontonagon County, and is the only incorporated place within the county.