County Maps US

Monroe County, Illinois

A county in Illinois, ranked 40 of 102 by population.

CountyFIPS 17133

Within Illinois
Monroe County, highlighted, with the areas around it. Boundaries generalised from the 2025 Census cartographic boundary file; neighbour fills carry no data. The dot marks Waterloo, the county seat, located via the 2025 Census Gazetteer.

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Population
34,840
-121 (-0.3%) since 2020
Land area
385.3
square miles. A further 3.3% of the total area is water
Population density
90.4
people per square mile, computed rather than published
County seat
Waterloo
Seat of county government.
Established
1816
209 years ago
Largest city
Waterloo city
Population 11,132.

Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025; Wikidata (CC0), Wikidata. Retrieved 30 August 2026.

Satellite view

Aerial imagery of Monroe County with its boundary drawn on top. A map shows you the shape; this shows you what is inside it, and which side of the line a place falls on.

Aerial view of Monroe County, Illinois from the USGS National Map, with its boundary outlined
Imagery: USGS National Map, USGS Imagery Only, public domain, requested from USGS by your browser when you reach this point on the page, at 701 by 800 pixels. The boundary is the 2025 Census cartographic boundary file, reprojected to Web Mercator to match the imagery. You can also open the USGS National Map viewer.

Where Monroe County ranks

Position among the 102 counties of Illinois, and among all 3,144 US counties and county-equivalents. Computed from the figures on this page.

MeasureValueIn IllinoisNationally
Population34,84040th of 1021,317th of 3,144
Land area385.3 sq mi75th of 1022,583rd of 3,144
Population density90.4 per sq mi25th of 102989th of 3,144
Median household income$102,8807th of 102147th of 3,143

Full data for Monroe County

Every figure below is attributed to the Census Bureau product it came from, with the vintage of that product.
MeasureValueSource and vintage
Population34,840Vintage 2025 Population Estimates
Population in 202034,961Vintage 2025 Population Estimates
2020 estimates base?34,956Vintage 2025 Population Estimates
Land area385.3 sq mi2025 Gazetteer Files
Water area13.0 sq mi2025 Gazetteer Files
Total area398.3 sq mi2025 Gazetteer Files
Population density?90.4 per sq miComputed: Vintage 2025 estimates divided by 2025 Gazetteer land area
Median household income$102,880American Community Survey 2020-2024 5-Year Estimates
Median home value$282,100American Community Survey 2020-2024 5-Year Estimates
Median age43.2American Community Survey 2020-2024 5-Year Estimates
Households13,851American Community Survey 2020-2024 5-Year Estimates
Housing units14,759American Community Survey 2020-2024 5-Year Estimates

Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025; US Census Bureau, American Community Survey, American Community Survey 2020-2024 5-Year Estimates, released 29 January 2026. Population density is computed here, not published: it divides the Vintage 2025 population estimate by the 2025 Gazetteer land area, which are two different Census products. Retrieved 30 August 2026.

How the population changed

The Census Bureau publishes the components behind an estimate, not just the total. For the year to July 2025:

Births minus deaths
-173
213 births, 386 deaths
Net migration
-93
More people moved out than moved in.

Source: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026. Retrieved 30 August 2026.

The 5 areas bordering Monroe County

Ordered by the length of the border actually shared, which the Census Bureau publishes in its county adjacency file and which no other county reference presents.

Bordering areaStateShared borderPopulation
St. Clair CountyIllinois37 mi250,708
Randolph CountyIllinois23 mi29,921
Jefferson CountyMissouri21 mi233,132
St. Louis CountyMissouri11 mi990,911
Ste. Genevieve CountyMissouri3.9 mi18,689

Source: US Census Bureau, County Adjacency File, 2025 County Adjacency File, released 15 September 2025. Retrieved 30 August 2026.

All 102 counties of Illinois

Open the interactive Illinois county map, or jump straight to any of them.

Data on this page last rebuilt 30 August 2026. How these figures are produced.