Mesa County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 155,703. The county seat and most populous municipality is Grand Junction. The county was named for the many large mesas in the area, including the Grand Mesa, which is the largest flat-topped mountain in the world.

Grand Junction is a home rule municipality that is the seat of government and largest city of Mesa County, Colorado, United States. Grand Junction's population was 65,560 at the 2020 United States Census, making it the most populous city in western Colorado and the 17th most populous Colorado municipality overall.