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's Lakewood.

Golden is a home rule city that is the county seat of Jefferson County, Colorado, United States. The city population was 20,399 at the 2020 United States Census. Golden lies along Clear Creek at the base of the Front Range of the Rocky Mountains. Founded during the Pike's Peak Gold Rush on June 16, 1859, the mining camp was originally named Golden City in honor of Thomas L. Golden. Golden City served as the capital of the provisional Territory of Jefferson from 1860 to 1861, and capital of the official Territory of Colorado from 1862 to 1867. In 1867, the territorial capital was moved about 12 miles (19 km) east to Denver City. Golden is now a part of the Denver–Aurora–Lakewood, CO Metropolitan Statistical Area and the Front Range Urban Corridor.