Warren County is a county located in the eastern portion of the U.S. state of Missouri. As of the 2020 census, the population was 35,532. The county is located on the north side of the Missouri River. Its county seat is Warrenton. The county was established on January 5, 1833, and was named for General Joseph Warren, who died in the Battle of Bunker Hill during the American Revolutionary War. Warren County is part of the St. Louis, MO-IL Metropolitan Statistical Area.