The Muddy River is a stream, about 29 miles (47 km) long, southeast of Mount St. Helens in the U.S. state of Washington. The Muddy River flows south–southeast and joins the Lewis River just above the Swift Reservoir. The Lewis River flows west and is a tributary to the Columbia River. The east flank of Mount St. Helens is within the watershed of Muddy Creek.
During the 1980 eruption of Mount St. Helens, lahars (volcanically-induced mud and debris flows) swept through its channel. About 135 miles (217 km) of stream channels around the volcano were affected by lahars.
The United States Forest Service maintains the Muddy River Picnic Site, next to the river and to Forest Road 25 in the Gifford Pinchot National Forest. Trails cross the river on its upper reaches.