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.
In 2010, the center of population of Colorado was located in Jefferson County.
The county's slogan is the "Gateway to the Rocky Mountains", and it is commonly nicknamed Jeffco. The name Jeffco is incorporated in the name of the Jeffco School District, the Jeffco Business Center Metropolitan District No. 1, and several businesses located in Jefferson County. Jeffco is also incorporated in the unofficial monikers of many Jefferson County agencies. The Rocky Mountain Metropolitan Airport operated by Jefferson County was previously known as the Jeffco Airport.
A major employer in Jefferson County is the large Coors Brewing Company in Golden. Also, the state-supported Colorado School of Mines is located in Jefferson County, offering programs in STEM topics such as mining, geology, chemistry, and engineering.