Union County is the southernmost county in the U.S. state of South Dakota. As of the 2020 census, the population was 16,811, making it the 13th most populous county in South Dakota. Its county seat has been Elk Point since April 30, 1865. Originally named Cole County, its name was changed to Union on January 7, 1864, because of Civil War sentiment.

Union County is part of the Sioux City, IA–NE–SD Metropolitan Statistical Area. The Progressive Farmer rated Union County second in the "2006 Best Place to Live Survey" in the U.S., because "its schools are good, its towns neat and its people friendly".

Founded on April 10, 1862, as Cole County, it was renamed Union County on January 7, 1864, when its boundaries were changed to encompass land previously part of neighboring Lincoln County. The county seat was moved from Richland to Elk Point on April 30, 1865.

Union County lies on the southeast corner of South Dakota. Its east boundary line abuts the west boundary line of the State of Iowa (across the Big Sioux River), and its south/southwest boundary line abuts the north boundary line of the State of Nebraska (across the Missouri River). The Brule Creek flows southeasterly across the central part of the county, emptying into the Big Sioux. The county terrain consists of rolling hills, devoted to agriculture except around built-up areas. The terrain slopes to the south and east; its highest point is near its northwest corner, at 1,509 ft (460 m) ASL. The county has a total area of 467 square miles (1,210 km), of which 461 square miles (1,190 km) is land and 6.6 square miles (17 km) (1.4%) is water. It is the fifth-smallest county in South Dakota by area.

• Burbank Lake (partial)
• McCook Lake
• Mud Lake

Historical population

Census	Pop.	Note	%±
1870	3,507		—
1880	6,813		94.3%
1890	9,130		34.0%
1900	11,153		22.2%
1910	10,676		−4.3%
1920	11,099		4.0%
1930	11,480		3.4%
1940	11,675		1.7%
1950	10,792		−7.6%
1960	10,197		−5.5%
1970	9,643		−5.4%
1980	10,938		13.4%
1990	10,189		−6.8%
2000	12,584		23.5%
2010	14,399		14.4%
2020	16,811		16.8%
2023 (est.)	17,183		2.2%
U.S. Decennial Census 1790-1960 1900-1990 1990-2000 2010-2020

As of the 2020 census, there were 16,811 people, 6,822 households, and 4,613 families in the county. The population density was 36.5 inhabitants per square mile (14.1/km). There were 7,215 housing units.

As of the 2010 census, there were 14,399 people, 5,756 households, and 4,043 families in the county. The population density was 31.3 inhabitants per square mile (12.1/km). There were 6,280 housing units at an average density of 13.6 per square mile (5.3/km). The racial makeup of the county was 95.5% white, 0.9% Asian, 0.7% black or African American, 0.6% American Indian, 0.1% Pacific islander, 0.7% from other races, and 1.6% from two or more races. Those of Hispanic or Latino origin made up 2.1% of the population. In terms of ancestry, 42.0% were German, 18.2% were Norwegian, 17.7% were Irish, 7.9% were English, 6.8% were Swedish, and 3.7% were American.

Of the 5,756 households, 32.8% had children under the age of 18 living with them, 58.9% were married couples living together, 7.5% had a female householder with no husband present, 29.8% were non-families, and 25.1% of all households were made up of individuals. The average household size was 2.49 and the average family size was 2.98. The median age was 40.2 years.

The median income for a household in the county was $59,889 and the median income for a family was $71,308. Males had a median income of $42,702 versus $31,993 for females. The per capita income for the county was $33,783. About 3.9% of families and 4.9% of the population were below the poverty line, including 4.3% of those under age 18 and 10.5% of those age 65 or over.

• Dakota Dunes
• Richland

• Richland

Union County voters were more politically centered in times past, but the county has selected the Republican Party candidate in every national election since 2000 (as of 2024).

United States presidential election results for Union County, South Dakota

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2024	6,160	69.40%	2,548	28.71%	168	1.89%
2020	5,944	67.13%	2,725	30.77%	186	2.10%
2016	5,290	66.99%	2,227	28.20%	380	4.81%
2012	4,698	61.85%	2,782	36.62%	116	1.53%
2008	4,310	55.97%	3,244	42.12%	147	1.91%
2004	3,987	56.57%	3,000	42.57%	61	0.87%
2000	3,265	56.57%	2,358	40.85%	149	2.58%
1996	2,234	42.90%	2,378	45.67%	595	11.43%
1992	1,784	35.00%	2,210	43.36%	1,103	21.64%
1988	1,907	41.91%	2,612	57.41%	31	0.68%
1984	2,431	51.98%	2,221	47.49%	25	0.53%
1980	2,788	55.16%	1,830	36.21%	436	8.63%
1976	2,297	47.26%	2,540	52.26%	23	0.47%
1972	2,271	46.90%	2,554	52.75%	17	0.35%
1968	2,212	49.70%	2,014	45.25%	225	5.06%
1964	1,727	37.91%	2,828	62.09%	0	0.00%
1960	2,688	53.56%	2,331	46.44%	0	0.00%
1956	2,636	52.94%	2,343	47.06%	0	0.00%
1952	3,393	67.35%	1,645	32.65%	0	0.00%
1948	2,205	49.38%	2,237	50.10%	23	0.52%
1944	2,501	54.87%	2,057	45.13%	0	0.00%
1940	3,116	54.44%	2,608	45.56%	0	0.00%
1936	1,845	31.89%	3,520	60.85%	420	7.26%
1932	1,381	27.86%	3,530	71.21%	46	0.93%
1928	2,415	53.19%	2,106	46.39%	19	0.42%
1924	1,665	45.04%	877	23.72%	1,155	31.24%
1920	1,942	66.17%	841	28.65%	152	5.18%
1916	1,108	44.95%	1,313	53.27%	44	1.78%
1912	0	0.00%	965	39.08%	1,504	60.92%
1908	1,392	56.63%	1,009	41.05%	57	2.32%
1904	1,813	68.31%	730	27.51%	111	4.18%
1900	1,571	53.00%	1,358	45.82%	35	1.18%
1896	1,297	46.24%	1,491	53.16%	17	0.61%
1892	860	43.30%	241	12.13%	885	44.56%

• National Register of Historic Places listings in Union County, South Dakota

• Union County Historical Society website
• 2nd Best Place to Live in 2006 from the Progressive Farmer website

42°50′N 96°39′W / 42.83°N 96.65°W / 42.83; -96.65