Douglas County is the eighth most populous of the 64 counties of the state of Colorado, in the United States. The county is located midway between Colorado's two largest cities: Denver and Colorado Springs. The United States Census Bureau that the county population was 285,465 in 2010 census, a 62.4% increase since the 2000 census, making Douglas County one of the fastest growing counties in the United States. Douglas County is part of the Denver-Aurora Metropolitan Statistical Area and the Denver-Aurora-Boulder Combined Statistical Area. The county seat is Castle Rock, named after a prominent castle tower-shaped butte just north of the town.
Douglas County is lightly wooded, mostly with ponderosa pine, with broken terrain characterized by mesas and small streams. Cherry Creek and Plum Creek rise in Douglas County and flow north toward Denver and into the South Platte River. Both were subject to flash flooding in the past, Plum Creek being partially responsible for the Denver flood of 1965. Cherry Creek is now dammed.
Most residents commute to workplaces elsewhere in the metropolitan area outside of the county. Suburban development is displacing the ranching economy of the county.
Douglas County was one of the original 17 counties created in the Colorado Territory by the Colorado Territorial Legislature on November 1, 1861. The county was named in honor of U.S. Senator Stephen A. Douglas of Illinois, who died five months before the county was created. The county seat was originally Franktown, but was moved to California Ranch in 1863, and then to Castle Rock in 1874. Although the county's boundaries originally extended eastward to the Kansas state border, in 1874 most of the eastern portion of the county became part of Elbert County.