Keystone is a town in the Black Hills region of Pennington County, South Dakota, United States. The population was 337 at the 2010 census. It had its origins in 1883 as a mining town, and has since transformed itself into a resort town, serving the needs of the millions of visitors to the Mount Rushmore National Memorial, which is located just beyond city limits. Keystone was heavily damaged in the 1972 Black Hills flood.