Boulder County is the sixth most populous county of the State of Colorado. The United States Census Bureau estimates that the county population was 280,440 in 2005. While the population of the area actually increased, the City and County of Broomfield separated from Boulder County in 2001. The county seat is the City of Boulder. Boulder County was one of the original 17 counties created by the Colorado legislature on November 1, 1861. It was named after Boulder City and Boulder Creek, so named because of the abundance of boulders in the area. It retains essentially the same borders now as it was created with then, although a small portion of its southeastern corner became part of the City and County of Broomfield in 2001.

As of the census² of 2000, there were 291,288 people, 114,680 households, and 68,808 families residing in the county. The population density was 151/km² (392/mi²). There were 119,900 housing units at an average density of 62/km² (162/mi²). The racial makeup of the county was 88.54% White, 0.88% Black or African American, 0.61% Native American, 3.06% Asian, 0.06% Pacific Islander, 4.67% from other races, and 2.18% from two or more races. 10.46% of the population were Hispanic or Latino of any race.