Ontonagon County ( ) is a county located in the Upper peninsula of the U.S. state of Michigan. As of the 2010 census, the population was 6,780, making it the third-least populous county in Michigan. The county seat is Ontonagon. The county was set off in 1843, and organized in 1848. Its territory had originally 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.