Butte County is a rural county located in the U.S. state of Idaho. As of the 2010 census, the population was 2,891, 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 Blaine County.Butte County is included in the Idaho Falls, ID Metropolitan Statistical Area. Much of the eastern part of the county is the vast Idaho National Laboratory, which extends eastward into neighboring counties.