County outline
Where Marion County is located
Area breakdown
Land accounts for 99.4% of Marion County’s measured area and water for 0.6%. Its combined area converts to 1,492.1 square kilometers or approximately 368,706 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 572.538 | 1,482.867 | 366,424.3 |
| Water | 3.565 | 9.233 | 2,281.6 |
| Total | 576.103 | 1,492.1 | 368,705.9 |
How Marion County compares
By original Census land area, Marion County ranks #40 of 102 in Illinois and #1749 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 1.121 times the Illinois county-equivalent median of 510.7 square miles. It accounts for 1% of the combined county land area in the state.
See the complete Illinois ranking · What counts as a county equivalent?
Population and density of Marion County
The Census Bureau estimates that 36,382 people lived in Marion County on July 1, 2025. That ranks #1,317 of 3,222 county equivalents nationally and #37 of 102 in Illinois. Since the April 1, 2020 estimates base of 37,740, the population has declined by 1,358 (−3.6%), compared with −0.8% for Illinois as a whole.
With 572.5 square miles of land, that is 64 people per square mile (25 per km²), placing Marion County #1,343 nationally for density and #37 in Illinois. The median county equivalent has 47 people per square mile; the Illinois median is 44. Put another way, there are about 10 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 37,656 | 37,383 | 36,860 | 36,660 | 36,501 | 36,382 |
| Change | — | −273 | −523 | −200 | −159 | −119 |
U.S. Census Bureau, Vintage 2025 population estimates. Estimates are revised each year and are not decennial census counts. Density divides the whole population by the whole county’s land area, so it describes an average, not how settlement is distributed within the county. Population sources and methods.
Cities and towns in Marion County
15 incorporated places lie at least partly in Marion County. Together they are home to 22,998 people (63.2%); the other 13,384 (36.8%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is Centralia city, with 9,598 residents in the county (the city extends into 3 other counties). The county seat, Salem city, has 7,035 residents.
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Centralia cityAlso in 3 other counties | 9,598 | −3.7% | 8.2 |
| Salem cityCounty seat | 7,035 | −3.6% | 6.991 |
| Sandoval village | 1,123 | −3.4% | 1.034 |
| Central City village | 1,067 | −3.5% | 0.565 |
| Odin village | 903 | −3.7% | 1.003 |
| Kinmundy city | 702 | −3.7% | 1.198 |
| Junction City village | 517 | −3% | 0.684 |
| Patoka village | 507 | −4.3% | 1.115 |
| Iuka village | 491 | −3.3% | 0.733 |
| Wamac cityAlso in 2 other counties | 395 | −2.9% | 1.415 |
| Alma village | 302 | −3.2% | 1.132 |
| Kell village | 169 | −2.9% | 1.002 |
| Vernon village | 97 | −4% | 0.882 |
| Walnut Hill village | 92 | −4.2% | 0.38 |
| Farina villageAlso in 1 other county | 0 | — | 1.457 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside Marion County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.
Metropolitan area
Marion County is a central county of the Centralia, IL Micropolitan Statistical Area. Marion County is the area’s only county equivalent, so the micro area has the same 572.5 square miles of land. By combined land it ranks #372 of 542 micropolitan areas, and #378 by population. It belongs to the St. Louis-St. Charles-Farmington, MO-IL Combined Statistical Area.
ZIP codes
15 ZIP Code Tabulation Areas have land in Marion County; 9 of them have most of their land inside the county, and the others mainly cover neighboring areas.
628016280762830628386284962851628536285462870628756288162882628896289262893
ZIP Code Tabulation Areas are the Census Bureau’s approximations of USPS ZIP Code service areas, from the 2020 Census relationship files. Lighter codes have less than half of their land in this county. Post-office-box and single-address ZIP Codes have no tabulation area.
Counties bordering Marion County
Marion County shares a land border with 6 county equivalents. It has more land than 4 of them. The largest neighbor is Fayette County (716.5 mi²) and the smallest is Clay County (468.4 mi²). Together with its neighbors, Marion County forms a block of 4,078.9 mi² of land, of which it holds 14%.
| Bordering county equivalent | Land mi² | Size vs. Marion | Compare |
|---|---|---|---|
| Fayette County | 716.5 | 1.3× larger | Compare |
| Wayne County | 713.8 | 1.2× larger | Compare |
| Jefferson County | 571.2 | about the same | Compare |
| Washington County | 562.6 | about the same | Compare |
| Clinton County | 473.9 | 1.2× smaller | Compare |
| Clay County | 468.4 | 1.2× smaller | Compare |
How big is Marion County compared with U.S. states?
Marion County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.8 times as much. Illinois’s county land could hold about 97 areas the size of Marion County.
Another way to picture it: the same land area would fill a square about 23.9 miles (38.5 km) on each side. Real county shapes are rarely square, so this is a scale reference rather than a width or driving distance.
State figures are the sums of each state’s county-equivalent land measurements in the same Census release, so states and counties are compared on the same basis. Published state totals can differ slightly because of rounding. Compare all states · Counties bigger than entire states.
What stands out about Marion County?
Marion County has more land than 1,473 of the other 3,221 county equivalents in the dataset.
A median is the middle county-equivalent measurement, not the average of the largest and smallest. All three bars use the same linear scale. Very small areas may appear as a thin sliver; the labels retain their values.
Does including water change its rank?
Using land plus water places Marion County at #1,860 nationally by total area, 111 places farther from the top than its land rank. Other county equivalents gain more area when their water measurements are included.
Its water share is 0.6%, compared with a national county-equivalent median of 1.2% — 0.6 percentage points lower. This describes the statistical geography’s land/water mix, not flood exposure or developable land.
Closest positions in the national land ranking
These entries sit immediately beside Marion County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1748 | Story County, Iowa | 572.5 | Compare |
| #1749 | Marion County, Illinois | 572.5 | This county |
| #1750 | York County, Nebraska | 572.5 | Compare |
Calculated from the same Census release as the area table. Total-area order uses original ALAND + AWATER; water shares and chart labels use published square miles. Calculation details.
Representative coordinates
The Census internal point is 38.648331, -88.920446. It represents a location inside the geography; it is not necessarily the centroid or the address of a courthouse.
Source and precision
U.S. Census Bureau, 2026 Gazetteer Files, Counties. Original ALAND: 1,482,866,781 m²; AWATER: 9,232,944 m². Rankings use ALAND; exact ties use ascending GEOID. Tables preserve Census-published square miles to three decimals, with conversions based on those values.
Common questions
How big is Marion County?
Marion County, Illinois has 572.5 square miles (1,482.9 km²) of land and 3.565 square miles of water, for a total area of 576.1 square miles. It ranks #1749 of 3,222 U.S. county equivalents by land area.
Is Marion County the largest county in Illinois?
No. Marion County ranks #40 of 102 in Illinois by land area. The largest is McLean County, with 1,183.2 square miles of land.
What counties border Marion County?
Marion County shares a land border with Fayette County, Jefferson County, Clinton County, Clay County, Wayne County, and Washington County.
What is the population of Marion County?
The Census Bureau estimates Marion County had 36,382 residents on July 1, 2025, ranking #1,317 of 3,222 county equivalents. That is 64 people per square mile of land.
What is the county seat of Marion County?
The county seat of Marion County is Salem.
What cities and towns are in Marion County?
Marion County has 15 incorporated places. The most populous five are Centralia city, Salem city, Sandoval village, Central City village, and Odin village; 13,384 residents live outside any incorporated place.
Is Marion County part of a metropolitan area?
Yes. It is a central county of the Centralia, IL Micropolitan Statistical Area.
How many ZIP codes are in Marion County?
15 ZIP Code Tabulation Areas have land in Marion County, and 9 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does Marion County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.8 times as much land.
How many acres of land are in Marion County?
The land measurement converts to approximately 366,424 acres. Including water, total area is approximately 368,706 acres. One square mile equals 640 acres.
Why might another source show a different area?
Check whether it reports land or total area, its Census vintage, and its rounding. Our guide explains how to reconcile county area figures.
Can this establish a property boundary?
No. Statistical measurements and generalized maps do not establish property lines. Consult the relevant local surveying or land-records authority.