Arco is a city in Butte County, Idaho, United States. The population was 1,026 at the 2000 census. The city is the county seat of Butte County. 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. Arco is located along the Big Lost River and is a gateway to the Lost River Valley.