Boulder is the home rule city in Boulder County, Colorado, United States, and its county seat. With a population of 108,250 at the 2020 census…Boulder is the home rule city in Boulder County, Colorado, United States, and its county seat. 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. It is the principal city of the Boulder metropolitan statistical area, which had 330,758 residents in 2020 and is part of the Front Range Urban Corridor.