Lake Francis Case is a large reservoir impounded by Fort Randall Dam on the Missouri River in south-central South Dakota, United States. The lake has an area of 102,000 acres (410 km2) and a maximum depth of 140 ft (43 m). Lake Francis Case has a length of approximately 107 mi (172 km) and has a shoreline of 540 mi (870 km). The lake is the eleventh-largest reservoir in the United States and is located within the counties of: Charles Mix, Gregory, Lyman, Brule, and Buffalo. The lake stretches from Pickstown, South Dakota upstream to Big Bend Dam.

Fort Randall Dam is a 2.03-mile-long (3 km) earthen dam which spans the Missouri River and impounds Lake Francis Case, the 11th-largest reservoir in the U.S. The dam joins Gregory and Charles Mix counties, South Dakota a distance of 880 river miles (1,416 km) upstream of St. Louis, Missouri, where the river joins the Mississippi River. The dam and hydroelectric power plant were constructed by and are currently operated by the U.S. Army Corps of Engineers.