Custer County is a rural mountain county in the center of the U.S. state of Idaho. As of the 2020 census, the population was 4,275, making it the fifth-least populous county in Idaho. The county seat is Challis. Established in 1881, the county was named for the General Custer Mine, where gold was discovered five years earlier. Custer County relies on ranching, mining, and tourism as its main resources.