Mahnomen County (/məˈnoʊmən/ mə-NOH-mən) is a county located in the U.S. state of Minnesota. As of the 2010 census, the population was 5,413. Its county seat is Mahnomen.

The entire county is part of the White Earth Indian Reservation. It is the only county in Minnesota entirely within an Indian reservation. The county, along with East Polk and Becker County, are one of the biggest cattle-raising areas in the northwestern part of Minnesota.