Jefferson County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 582,910, making it the fourth-most populous county in Colorado. The county seat is Golden, and the most populous city is Lakewood. Jefferson County is included in the Denver-Aurora-Lakewood, CO Metropolitan Statistical Area. Located along the Front Range of the Rocky Mountains, Jefferson County is adjacent to the state capital of Denver.