How many people live in St. Clair County, IL?
Updated Apr. 6, 2026Refreshed annually
In 2024, there were about 251,100 people living in St. Clair County, IL. That makes St. Clair County the ninth-most populous county in Illinois. It is home to 2% of people living in the state.
251K
population of St. Clair County, IL (2024)
2%
County share of the Illinois population (2024)
About 251,100 people lived in St. Clair County, IL in 2024.
Population estimates, 2000–2024
What are St. Clair County, IL’s age demographics?
About 46,500 St. Clair County residents are aged 65 and older in 2024. That represents 18.5% of its population. For comparison, the age group represents 17.9% of Illinois’ population and 18% in the US overall.
Another 56,000 St. Clair County residents, or 22.3% of the population, were children under 18. For comparison, the age group represents 21.2% of Illinois’s population and 21.6% in the US overall.
A lower share of St. Clair County, IL's population is aged 18–24 than the US as a whole.
Population estimates by age group, (2024)
What are St. Clair County, IL’s racial and ethnic demographics?
Non-Hispanic white residents make up 59.7% of the St. Clair County population, higher than the national share of 57.5%. St. Clair County also has a larger share of people identifying as non-Hispanic Black or African American (30.2%) than the US overall (12.6%).
In contrast, St. Clair County’s Hispanic population accounts for 5.3%, below the 20% nationwide, and the non-Hispanic Asian population makes up 1.8% of the county, compared with 6.5% nationally.
Racial and ethnic make up differ based on location. These figures reflect how people identified their race alone (not in combination with other races), and separate out Hispanic or Latino ethnicity, following Census reporting standards.
St. Clair County, IL has a higher share of a non-Hispanic Black or African American population than Illinois as a whole.
Population estimates by race and ethnicity, (2024)
The largest city or town in St. Clair County, IL is Belleville with about 40,600 residents in 2024.
Cities and towns in St. Clair County, IL ranked by population
Population (2024), includes only portions within county
| City or town | Population | |
|---|---|---|
| 1. | Belleville | 40,560 |
| 2. | O'Fallon (pt.) | 23,515 |
| 3. | East St. Louis | 17,808 |
| 4. | Cahokia Heights (pt.) | 17,052 |
| 5. | Fairview Heights (pt.) | 14,977 |
| 6. | Swansea (pt.) | 13,774 |
| 7. | Mascoutah (pt.) | 9,118 |
| 8. | Stookey | 8,859 |
| 9. | O'Fallon (pt.) | 8,654 |
| 10. | Shiloh (pt.) | 7,279 |
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Methodology
USAFacts standardizes data, in areas such as time and demographics, to make it easier to understand and compare.
The analysis was generated with the help of AI and reviewed by USAFacts for accuracy.
Page sources
USAFacts endeavors to share the most up-to-date information available. We sourced the data on this page directly from government agencies; however, the intervals at which agencies publish updated data vary.
