Foss Reservoir, also known as Foss Lake, is in Custer County, Oklahoma on the Washita River, about 15 miles (24 km) west of Clinton, Oklahoma. The reservoir was constructed during 1958–1961 by the U. S. Bureau of Reclamation. The project was known originally as the Washita Basin Project. The lake and dam were named for the community of Foss, Oklahoma, about 6 miles (9.7 km) south of the site. Their primary purpose is to regulate flow of the river and to provide water for the cities of Bessie, Clinton, Cordell and Hobart. It is western Oklahoma's largest lake and lies entirely within Foss State Park.

The reservoir has a surface area of 8,800 acres (3,600 ha) and a shoreline of 63 miles (101 km). The capacity of the reservoir is 436,812 acre-feet (538,800,000 m). The reservoir serves a catchment area of 1,496 square miles (3,870 km). The reservoir has a mean depth of 23 feet (7.0 m) and a maximum depth of 89 feet (27 m).

Dam construction began in October, 1958. The dam is 142 feet (43 m) high, 38 feet (12 m) wide (at the crest) and 18,130 feet (5,530 m) long.

Foss State Park encompasses 1,749 acres (7,080,000 m) of land and contains 8 campgrounds, swimming beach, 6 boat ramps, a playground, marina, and a restaurant. It also has 19 miles (31 km) of equestrian and multi-use trails. Foss State Park is often mentioned as a target for sale or closure to help close the state's current budget deficit. Already the state has sold a herd of bison as "surplus property." The bison had recently been moved into Foss State Park from another location in western Oklahoma.

The water quality in Lake Foss is extremely hard. The Bureau of Reclamation built one of the first electrodialysis plants in the United States to process the water before it is delivered to users. The Oklahoma Department of Environmental Quality constructed a new, updated plant to replace the original in 2002. The new plant, built by Ionics, Inc., would also increase production of potable water for its municipal customers from 3 million to 4.5 million U.S. gallons per day.

Water is transported from the dam to consumers via three pumping stations and 50.8 miles (81.8 km) of aqueducts.