County outline
Where Union County is located
Area breakdown
Land accounts for 98.9% of Union County’s measured area and water for 1.1%. Its combined area converts to 1,209.9 square kilometers or approximately 298,963 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 461.977 | 1,196.515 | 295,665.3 |
| Water | 5.152 | 13.344 | 3,297.3 |
| Total | 467.129 | 1,209.859 | 298,962.6 |
How Union County compares
By original Census land area, Union County ranks #62 of 66 in South Dakota and #2223 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 0.508 times the South Dakota county-equivalent median of 909.3 square miles. It accounts for 0.6% of the combined county land area in the state.
See the complete South Dakota ranking · What counts as a county equivalent?
Population and density of Union County
The Census Bureau estimates that 17,402 people lived in Union County on July 1, 2025. That ranks #2,024 of 3,222 county equivalents nationally and #13 of 66 in South Dakota. Since the April 1, 2020 estimates base of 16,813, the population has grown by 589 (+3.5%), compared with +5.5% for South Dakota as a whole.
With 462 square miles of land, that is 38 people per square mile (15 per km²), placing Union County #1,804 nationally for density and #8 in South Dakota. The median county equivalent has 47 people per square mile; the South Dakota median is 5.7. Put another way, there are about 17 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 16,855 | 16,881 | 17,079 | 17,149 | 17,373 | 17,402 |
| Change | — | +26 | +198 | +70 | +224 | +29 |
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 Union County
5 incorporated places lie at least partly in Union County. Together they are home to 8,082 people (46.4%); the other 9,320 (53.6%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is North Sioux City city, with 3,036 residents in the county. The county seat, Elk Point city, has 2,080 residents.
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| North Sioux City city | 3,036 | −0.4% | 3.335 |
| Elk Point cityCounty seat | 2,080 | −3.1% | 1.549 |
| Beresford cityAlso in 1 other county | 1,742 | −3.8% | 1.784 |
| Alcester city | 776 | −5.9% | 1.029 |
| Jefferson city | 448 | −5.3% | 0.51 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside Union County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.
Metropolitan area
Union County is a central county of the Sioux City, IA-NE-SD Metropolitan Statistical Area. Its 3 county equivalents have 145,738 residents on 1,599.3 square miles of land; Union County accounts for 28.9% of the land and 11.9% of the residents. By combined land it ranks #206 of 393 metropolitan areas, and #302 by population. It belongs to the Sioux City-Le Mars, IA-NE-SD Combined Statistical Area.
| County equivalent | Population 2025 | Land mi² |
|---|---|---|
| Woodbury CountyIowa | 106,649 | 872.9 |
| Dakota CountyNebraska | 21,687 | 264.4 |
| Union County | 17,402 | 462 |
ZIP codes
9 ZIP Code Tabulation Areas have land in Union County; 4 of them have most of their land inside the county, and the others mainly cover neighboring areas.
510015102357001570045701057025570345703857049
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 Union County
Union County shares a land border with 7 county equivalents, including 5 in Iowa and Nebraska. It has more land than 2 of them. The largest neighbor is Woodbury County (872.9 mi²) and the smallest is Dakota County (264.4 mi²). Together with its neighbors, Union County forms a block of 4,695 mi² of land, of which it holds 9.8%.
| Bordering county equivalent | Land mi² | Size vs. Union | Compare |
|---|---|---|---|
| Woodbury CountyIowa | 872.9 | 1.9× larger | Compare |
| Plymouth CountyIowa | 862.8 | 1.9× larger | Compare |
| Sioux CountyIowa | 767.9 | 1.7× larger | Compare |
| Lincoln County | 577.3 | 1.2× larger | Compare |
| Dixon CountyNebraska | 475.6 | about the same | Compare |
| Clay County | 412 | 1.1× smaller | Compare |
| Dakota CountyNebraska | 264.4 | 1.7× smaller | Compare |
How big is Union County compared with U.S. states?
Union County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 2.2 times as much. South Dakota’s county land could hold about 164 areas the size of Union County.
Another way to picture it: the same land area would fill a square about 21.5 miles (34.6 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 Union County?
Union County has more land than 999 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 Union County at #2,295 nationally by total area, 72 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 1.1%, compared with a national county-equivalent median of 1.2%. The two percentages are within 0.1 percentage points of each other. 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 Union County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #2222 | Adams County, Mississippi | 462.3 | Compare |
| #2223 | Union County, South Dakota | 462 | This county |
| #2224 | Wythe County, Virginia | 462 | 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 42.831106, -96.650829. 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: 1,196,514,489 m²; AWATER: 13,342,856 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 Union County?
Union County, South Dakota has 462 square miles (1,196.5 km²) of land and 5.152 square miles of water, for a total area of 467.1 square miles. It ranks #2223 of 3,222 U.S. county equivalents by land area.
Is Union County the largest county in South Dakota?
No. Union County ranks #62 of 66 in South Dakota by land area. The largest is Meade County, with 3,470.9 square miles of land.
What counties border Union County?
Union County shares a land border with Plymouth County, Iowa, Clay County, Lincoln County, Dixon County, Nebraska, Sioux County, Iowa, Dakota County, Nebraska, and Woodbury County, Iowa.
What is the population of Union County?
The Census Bureau estimates Union County had 17,402 residents on July 1, 2025, ranking #2,024 of 3,222 county equivalents. That is 38 people per square mile of land.
What is the county seat of Union County?
The county seat of Union County is Elk Point.
What cities and towns are in Union County?
Union County has 5 incorporated places. The most populous are North Sioux City city, Elk Point city, Beresford city, Alcester city, and Jefferson city; 9,320 residents live outside any incorporated place.
Is Union County part of a metropolitan area?
Yes. It is a central county of the Sioux City, IA-NE-SD Metropolitan Statistical Area, which has 3 county equivalents and 145,738 residents.
How many ZIP codes are in Union County?
9 ZIP Code Tabulation Areas have land in Union County, and 4 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does Union County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 2.2 times as much land.
How many acres of land are in Union County?
The land measurement converts to approximately 295,665 acres. Including water, total area is approximately 298,963 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.