Boulder County is the sixth most populous of the 64 counties of the state of Colorado of the United States. The most populous municipality in the county and the county seat is the City of Boulder. The United States Office of Management and Budget (OMB) has designated Boulder County as the Boulder, CO Metropolitan Statistical Area, a component of the Denver-Aurora-Boulder, CO Combined Statistical Area.
The United States Census Bureau that the county population was 294,567 in 2010 census. This was a 8.4% gain between 2010 and 2000, but a gain of 3.95% since 2000 excluding the area transferred to the City and County of Broomfield. Boulder Metropolitan Statistical Area is the 161st most populous metropolitan statistical area in the United States.
Boulder County was one of the original 17 counties created by the Territory of Colorado on November 1, 1861. The county was named for Boulder City and Boulder Creek, so named because of the abundance of boulders in the area. Boulder County retains essentially the same borders as in 1861, although a of its southeastern corner and its approximate population of 40,000 became part of the City and County of Broomfield in 2001.