Butte County is a rural county located in the U.S. state of Idaho. As of the 2020 census, the population was 2,574, making it the third-least populous county in Idaho. Its county seat and largest city is Arco. The county was established in 1917 from parts of Bingham, Blaine, and Jefferson counties. The county gained territory in the Clyde area from Custer County in 1937 to reach its present boundary.