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.
Originally known as Root Hog, the original town site was five miles (8 km) south at the junction of two stagecoach lines (Blackfoot-Wood River and Blackfoot-Salmon). A suspension bridge that crossed the Big Lost River funneled traffic through the settlement. The town leaders applied to the U.S. Post Office for the town name of "Junction".
The Postmaster General thought the name too common and suggested that the place be named Arco for Georg von Arco (1869–1940) of Germany who was visiting Washington, D.C. at the time. Georg von Arco was an inventor and a pioneer in the field of radio transmission and would become the lead engineer of Telefunken, a German company founded in 1903 that produced radio vacuum tubes. The town later moved four miles southeast when the stage station was moved to Webb Springs at Big Southern Butte. When the Oregon Short Line Railroad arrived from Blackfoot in 1901 the stage lines became obsolete and the town of Arco moved northwest to its present site.
Arco was the first community in the world ever to be lit by electricity generated solely by nuclear power. This occurred for about an hour on July 17, 1955, powered by Argonne National Laboratory’s BORAX-III reactor at the nearby National Reactor Testing Station (NRTS), now the Idaho National Laboratory.