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.