Foss River Falls is a drop on the West Fork Foss River in the Alpine Lakes Wilderness Area, King County, Washington. Its sources are at Delta Lake, and the -wide drop flows year-round. The waterfall is split into two parts. Each part is split into several tiers.
The upper section starts off by plunging , then cascades , then spits and drops over a plunge and a horsetail. Then the waters gather and plunge over a drop, and then cascade 150 more feet (46 m) down the gorge. The upper part of the falls, in total, drops about . The upper falls are located at .
The lower section begins with some small drops that total about , then drops feet down granite steps and flows into Trout Lake. The falls' flow rate at its base is about per second. The lower falls are located at .