The Foss River is a stream in the U.S. state of Washington. It has two main forks, the East Fork Foss River and the West Fork Foss River. The main stem is formed by the confluence of the two forks. The river and its forks rise near in the Alpine Lakes Wilderness area of Mount Baker-Snoqualmie National Forest in the Cascade Mountains and flows generally north, joining the Tye River to form the South Fork Skykomish River.