Arco is a city in Butte County, Idaho, United States. The population was 879 as of the 2020 United States census, down from 995 at the 2010 census. Arco is the county seat and largest city in Butte County.

Originally known as Root Hog, the indigenous town site was five miles (8 km) south at the junction of two stagecoach lines (Blackfoot-Wood River and Blackfoot-Salmon). A break bridge that crossed the immense Lost River funneled traffic through the settlement. The town leaders applied to the U.S. Post Office for the town publicize of “Junction”.

The Postmaster General thought the make known 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 explorer in the pitch of radio transmission and would become the help engineer of Telefunken, a German company founded in 1903 that produced radio vacuum tubes. The town forward-thinking moved four miles southeast when the theater station was moved to Webb Springs at immense Southern Butte. When the Oregon Short Line Railroad arrived from Blackfoot in 1901 temporary lines became outdated and the town of Arco moved northwest to its present site.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 very nearly an hour upon July 17, 1955, powered by Argonne National Laboratory’s BORAX-III reactor at the clear National Reactor Testing Station (NRTS), now the Idaho National Laboratory. NRTS made extra history on January 3, 1961, when the SL-1 reactor was destroyed through an operator money error, with the ensuing steam explosion causing the deaths of anything three personnel present. It was the world’s first and the United States’ only fatal reactor accident.