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. The Beaverhead becomes the Jefferson River 2 mi (3.2 km) downstream where it joins the Big Hole River.
The river has also been known by these various names: Pah-mamar-roi, Pak-sam-ma-oi, Pashmaroi, Passamari, Philanthrophy River, Stinking Water Creek, Stinking Water River, Stinkingwater River.
The Ruby River provides excellent fly fishing for anglers despite some battles with whirling disease and a massive fish kill in the lower stretches in 1994.
The upper stretch of the river, above Ruby Reservoir, has excellent fly fishing for rainbows, cutthroats and grayling. The lower stretch of the river, below Ruby Dam, provides excellent brown trout fishing. The average size of browns runs between 10-14 inches, with some fish reaching 18-20 inches. Because of limited access, fishing pressure on the lower river is light.