Ontonagon County is a county in the Upper Peninsula 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.
The county is named after the Ontonagon River. The name is said to be loosely derived from an Ojibwe language word noojitoon ziibi, meaning "hunting river." A French transliteration, Nantounagon, identified the river on a 1670 French map. Alternatively, and perhaps more accurately, it is said to be derived from the Ojibwa onagon, which means "dish" or "bowl."
According to the US Census Bureau, the county has a total area of 3,741 square miles, of which 1,311 square miles is land and 2,430 square miles (65%) is water. It is the third-largest county in Michigan by area.