Ravalli County is a county located in the southwestern part of the U.S. state of Montana. As of the 2010 census, the population was 40,212. Its county seat is Hamilton.

Ravalli County is part of a north/south mountain valley bordered by the Sapphire Mountains on the East and the Bitterroot Mountains on the West. It is often referred to as the Bitterroot Valley, which is named for the Bitterroot Flower. The county is entirely on the Pacific Ocean side of the Continental Divide, which follows the Idaho-Montana border from Wyoming until Ravalli County. Here it turns east into Montana, between Chief Joseph Pass and Lost Trail Pass, and follows the Ravalli County-Beaverhead County border.