The Ruby River is a tributary of the Beaverhead River, approximately 76 mi (122 km) long, in southwestern Montana in the United States. It rises in the Beaverhead National Forest in southwestern Madison County between the Snowcrest Range and the Gravelly Range. It flows north through the Ruby River Reservoir, past Alder, then northwest, flowing between the Tobacco Root Mountains to the northeast and the Ruby Range to the southwest. It joins the Beaverhead near Twin Bridges.