The Cosumnes River is a river in northern California in the United States. It rises on the western slope of the Sierra Nevada and flows approximately 52.5 miles into the Central Valley, emptying into the Mokelumne River in the Sacramento-San Joaquin Delta.
The Cosumnes is one of very few rivers in the western Sierra without major dams. The Nature Conservancy's Cosumnes River Preserve is located just upstream from the Delta. Towns and cities along the Cosumnes River include Plymouth, Rancho Murieta, Sloughhouse, Wilton, Elk Grove, and Galt.
The California Office of Environmental Health Hazard Assessment has issued an advisory regarding fish caught from the body of water.