Keystone is an unincorporated census-designated place in Hillsborough County, Florida, United States. The Keystone area was first settled in the middle 1800s by the W.M. Mobley Family who migrated from Savannah, Georgia. The region has evolved from citrus and farming into a rural retreat of many lake homes and ranches. The population was 24,039 at the 2010 census, up from 14,627 at the 2000 census. The ZIP Codes encompassing the CDP area are 33556 and 33558.