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 Mount Rushmore National Memorial, which is located just beyond city limits.The town took its name from a local mine, which most likely was named after the keystone Masonic symbol.