Keystone, SD -- U.S. town in South Dakota Population (2000): 311 Housing Units (2000): 209 Land area (2000): 2.864807 sq. miles (7.419816 sq. km) Water area (2000): 0.000000 sq. miles (0.000000 sq. km) Total area (2000): 2.864807 sq. miles (7.419816 sq. km) FIPS code: 33820 Located within: South Dakota (SD), FIPS 46 Location: 43.893744 N, 103.426080 W ZIP Codes (1990): 57751 Note: some ZIP codes may be omitted esp. for suburbs. Headwords: Keystone, SD Keystone