Arco is a city in Butte County, Idaho, United States. The population was 995 at the 2010 census. Arco is the county seat and largest city in Butte County. Arco is located along the Big Lost River and is a gateway to the Lost River Range from the Snake River Plain. Craters of the Moon National Monument is located along U.S. Route 20, southwest of the city. The Idaho National Laboratory (INL) is located east of Arco.