Monmouth
- County:
- Warren County
- County Seat:
- Yes
- Area (mi²):
- 4.242
- State:
- Illinois
Monmouth is a city located in Warren County, Illinois. Monmouth has a 2026 population of 8,639. It is also the county seat of Warren County. Monmouth is currently growing at a rate of 0.42% annually but its population has decreased by -3.11% since the most recent census, which recorded a population of 8,916 in 2020.