COUNTY · IL
Marion County, Illinois
Marion County is a county in Illinois with FIPS code 17121 and an estimated population of 36,673 (Census 2023), 37th largest of 102 in the state.
County facts
- Name
- Marion County
- State
- Illinois
- Type
- county
- FIPS code
- 17121
- State FIPS
- 17
- Population (2023)
- 36,673
- Change since 2020
- -2.6%
- State rank
- 37 of 102
Population is the Census Bureau’s Vintage 2023 estimate.
Questions
What is the FIPS code for Marion County?
Marion County, Illinois has the five-digit FIPS code 17121: 17 for the state and 121 for the county.
What state is Marion County in?
Marion County is in Illinois.
How do I know if an address is in this county?
Use the address lookup. ZIP codes and city names can cross county lines, so an address is the reliable check.
Sources: U.S. Census Bureau geographic reference files; Census Bureau Population Estimates, Vintage 2023. Last reviewed 2026-10-27.