County Maps US

Union County, Ohio

A county in Ohio, ranked 34 of 88 by population.

CountyFIPS 39159

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

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Population
73,446
+10,377 (+16.5%) since 2020
Land area
431.7
square miles. A further 1.2% of the total area is water
Population density
170
people per square mile, computed rather than published
County seat
Marysville
Seat of county government.
Established
1820
205 years ago
Largest city
Marysville city
Population 30,190.

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 Union 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 Union County, Ohio 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 605 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 Union County ranks

Position among the 88 counties of Ohio, and among all 3,144 US counties and county-equivalents. Computed from the figures on this page.

MeasureValueIn OhioNationally
Population73,44634th of 88764th of 3,144
Land area431.7 sq mi45th of 882,351st of 3,144
Population density170 per sq mi30th of 88629th of 3,144
Median household income$112,3222nd of 8880th of 3,143

Full data for Union County

Every figure below is attributed to the Census Bureau product it came from, with the vintage of that product.
MeasureValueSource and vintage
Population73,446Vintage 2025 Population Estimates
Population in 202063,069Vintage 2025 Population Estimates
2020 estimates base?62,795Vintage 2025 Population Estimates
Land area431.7 sq mi2025 Gazetteer Files
Water area5.2 sq mi2025 Gazetteer Files
Total area436.9 sq mi2025 Gazetteer Files
Population density?170 per sq miComputed: Vintage 2025 estimates divided by 2025 Gazetteer land area
Median household income$112,322American Community Survey 2020-2024 5-Year Estimates
Median home value$344,900American Community Survey 2020-2024 5-Year Estimates
Median age38.5American Community Survey 2020-2024 5-Year Estimates
Households24,232American Community Survey 2020-2024 5-Year Estimates
Housing units25,158American 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
+344
817 births, 473 deaths
Net migration
+1,119
More people moved in than moved out.

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

The 7 areas bordering Union 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
Delaware CountyOhio25 mi242,032
Logan CountyOhio19 mi46,302
Madison CountyOhio16 mi46,985
Marion CountyOhio13 mi65,115
Champaign CountyOhio11 mi38,820
Hardin CountyOhio5.4 mi30,072
Franklin CountyOhio4.4 mi1,361,536

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

All 88 counties of Ohio

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

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