Jefferson County (Jeffco) is one of the 64 counties in the U.S. state of Colorado. As of the 2010 census, the population was 534,543, 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.