Beaverhead County is the largest county by area in the U.S. state of Montana. As of the 2010 census, the population was 9,246. Its county seat is Dillon. The county was founded in 1865. Much of the perimeter of the county is the Continental Divide, including all of its border with the state of Idaho. The divide heads east into Montana at the county border with Ravalli County, between Lost Trail Pass and Chief Joseph Pass.