Marion
- County:
- Williamson County
- County Seat:
- Yes
- Area (mi²):
- 14.403
- State:
- Illinois
Marion is a city located in Williamson County, Illinois. Marion has a 2026 population of 17,021. It is also the county seat of Williamson County. Marion is currently growing at a rate of 0.97% annually and its population has increased by 1.13% since the most recent census, which recorded a population of 16,831 in 2020.