Ontonagon County (/ˌɒntəˈnɑːɡən/ ON-tə-NAH-gən) 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.