Boulder is the Home Rule City that is the seat of Boulder County, Colorado, United States. [1] With a population of 108,250 at the 2020 census, it is the most populous city in the county and the 12th-most populous city in Colorado. [7] Boulder is the principal city of the Boulder, CO Metropolitan Statistical Area, which had 330,758 residents in 2020 and is part of the Front Range Urban Corridor.