Marquette County is a county located in the Upper Peninsula of the U.S. state of Michigan. As of the 2020 census, the population was 66,017. It is the most populous county in the Upper Peninsula. The county seat is Marquette. The county is named for Father Marquette, a Jesuit missionary. It was set off in 1843 and organized in 1851. Marquette County comprises the Marquette, MI micropolitan statistical area.