CITY · IL

What county is Chicago, IL in?

Chicago, IL is mostly in Cook County. County FIPS code 17031, population 5,087,072 (2023 estimate).

County details

City: Chicago, Illinois
Primary county: Cook County
FIPS code: 17031
County population: 5,087,072

Some cities extend into more than one county. Chicago is placed in the county that contains its center. For a specific address, use the address lookup.

Questions

What county is Chicago in?

Chicago, IL is mostly in Cook County.

What is the county seat of Cook County?

County seats are not part of the data on this site. Check the county’s official website.

Can a city be in two counties?

Yes. Large cities such as Houston and Atlanta cross county lines. The address lookup shows the exact county for any street address.

Sources: U.S. Census Bureau geographic reference files; FCC Area API; Census Bureau Population Estimates, Vintage 2023. Last reviewed 2026-10-27.