Monmouth is a city in and the county seat of Warren County, Illinois, United States. The population was 8,902 at the 2020 census, down from 9,444 in 2010. It is the home of Monmouth College and contains Monmouth Park, Harmon Park, North Park, Warfield Park, West Park, South Park, Garwood Park, Buster White Park and the Citizens Lake& Campground. It is the host of the Prime Beef festival, held annually the week after Labor Day.