Union County is a county located on the central southern border of the U.S. state of Arkansas. As of the 2020 census, the population was 39,054. The county seat is El Dorado. The county was formed on November 2, 1829, and named in recognition of the citizens' petition for a new county, which said that they were petitioning "in the spirit of Union and Unity." The county is directly adjacent to the south to Union Parish in the state of Louisiana.

The El Dorado, AR Micropolitan Statistical Area includes all of Union County.

Called by boosters the "Queen City of South Arkansas", El Dorado was at the heart of the 1920s oil boom in South Arkansas. More recently, the city has been called "Arkansas's Original Boomtown," as it emphasizes its historic assets for heritage tourism. The chemical and timber industries became important during and after World War II, and still have a place in the economy.

Union County was formed on November 2, 1829, from portions of Clark and Hempstead counties. Areas along the waterways were originally developed for cotton plantations in the antebellum years, and planters depended on large groups of enslaved African-American workers to generate their profits.

On January 10, 1921, Dr. Samuel T. Busey hit oil with a well about a mile south of El Dorado, leading to an oil boom that attracted thousands of workers and speculators. His first well produced for fewer than two months, but by 1923, "El Dorado boasted fifty-nine oil contracting companies, thirteen oil distributors and refiners, and twenty-two oil production companies. The city was flooded with so many people that no bed space was available for them, leading to whole neighborhoods of tents and hastily constructed shacks to be erected throughout the city. The city's population reached a high of nearly 30,000 in 1925 during the boom before dropping to 16,241 by 1930 and rising to 25,000 by 1960." Oil production fell markedly in the early 1930s, in part due to companies' financial difficulties during the Great Depression. It recovered later in the decade.

During World War II, chemical plants were established in the county, but their production declined after the war. Exploitation of Arkansas forests led to growth in the timber industry in the county. Oil, chemical and timber industries are still important to the economy, although in lesser proportion.

According to the U.S. Census Bureau, the county has a total area of 1,055 square miles (2,730 km), of which 1,039 square miles (2,690 km) is land and 16 square miles (41 km) (1.5%) is water. It is the largest county by area in Arkansas. Union County, along with Columbia County, has the largest bromine reserve in the United States. The lowest point in the state of Arkansas, also the lowest point in all landlocked U.S. states, is located on the Ouachita River in Union County and Ashley County, where it flows out of Arkansas and into Louisiana.

• Ouachita County (northwest)
• Calhoun County (north)
• Bradley County (northeast)
• Ashley County (east)
• Morehouse Parish, Louisiana (southeast)
• Union Parish, Louisiana (south)
• Claiborne Parish, Louisiana (southwest)
• Columbia County (west)

• Felsenthal National Wildlife Refuge (part)

Historical population

Census	Pop.	Note	%±
1830	640		—
1840	2,889		351.4%
1850	10,298		256.5%
1860	12,288		19.3%
1870	10,571		−14.0%
1880	13,419		26.9%
1890	14,977		11.6%
1900	22,495		50.2%
1910	30,723		36.6%
1920	29,691		−3.4%
1930	55,800		87.9%
1940	50,461		−9.6%
1950	49,686		−1.5%
1960	49,518		−0.3%
1970	45,428		−8.3%
1980	48,573		6.9%
1990	46,719		−3.8%
2000	45,629		−2.3%
2010	41,639		−8.7%
2020	39,054		−6.2%
2023 (est.)	37,397		−4.2%
U.S. Decennial Census 1790–1960 1900–1990 1990–2000 2010

Union County racial composition

Race	Num.	Perc.
White (non-Hispanic)	22,904	58.65%
Black or African American (non-Hispanic)	12,670	32.44%
Native American	108	0.28%
Asian	273	0.7%
Pacific Islander	16	0.04%
Other/Mixed	1,329	3.4%
Hispanic or Latino	1,754	4.49%

As of the 2020 United States census, there were 39,054 people, 15,726 households, and 10,562 families residing in the county.

As of the 2000 census, there were 45,629 people, 17,989 households, and 12,646 families residing in the county. The population density was 44 people per square mile (17 people/km). There were 20,676 housing units at an average density of 20 per square mile (7.7/km). The racial makeup of the county was 66.15% White, 31.97% Black or African American, 0.24% Native American, 0.40% Asian, 0.01% Pacific Islander, 0.46% from other races, and 0.77% from two or more races. 1.14% of the population were Hispanic or Latino of any race.

There were 17,989 households, out of which 32.20% had children under the age of 18 living with them, 51.30% were married couples living together, 15.20% had a female householder with no husband present, and 29.70% were non-families. 26.90% of all households were made up of individuals, and 12.10% had someone living alone who was 65 years of age or older. The average household size was 2.48 and the average family size was 3.00.

In the county, the population was spread out, with 25.90% under the age of 18, 8.30% from 18 to 24, 27.00% from 25 to 44, 22.70% from 45 to 64, and 16.10% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 91.60 males. For every 100 females age 18 and over, there were 86.00 males.

The median income for a household in the county was $29,809, and the median income for a family was $36,805. Males had a median income of $31,868 versus $19,740 for females. The per capita income for the county was $16,063. About 14.70% of families and 18.70% of the population were below the poverty line, including 25.80% of those under age 18 and 14.30% of those age 65 or over.

Over the past few election cycles, Union County has trended heavily towards the GOP. The last Democrat (as of 2020) to carry this county was Bill Clinton (a native Arkansan) in 1996.

United States presidential election results for Union County, Arkansas

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2020	10,478	63.09%	5,584	33.62%	545	3.28%
2016	10,456	61.89%	5,855	34.66%	583	3.45%
2012	10,699	62.29%	6,196	36.07%	282	1.64%
2008	10,677	62.15%	6,190	36.03%	312	1.82%
2004	10,502	58.89%	7,071	39.65%	259	1.45%
2000	8,647	55.40%	6,261	40.11%	701	4.49%
1996	6,053	37.97%	8,373	52.53%	1,514	9.50%
1992	7,305	39.32%	8,786	47.29%	2,489	13.40%
1988	10,581	61.32%	5,931	34.37%	744	4.31%
1984	12,333	65.74%	6,208	33.09%	218	1.16%
1980	9,401	55.10%	6,852	40.16%	810	4.75%
1976	7,918	48.93%	8,257	51.03%	7	0.04%
1972	11,925	76.72%	3,531	22.72%	87	0.56%
1968	4,919	28.60%	4,426	25.74%	7,853	45.66%
1964	8,472	54.38%	6,948	44.60%	160	1.03%
1960	5,631	41.51%	6,500	47.92%	1,434	10.57%
1956	5,059	39.75%	7,055	55.44%	612	4.81%
1952	5,266	41.11%	7,515	58.67%	29	0.23%
1948	1,039	12.46%	5,588	67.01%	1,712	20.53%
1944	833	15.26%	4,624	84.70%	2	0.04%
1940	489	9.15%	4,842	90.59%	14	0.26%
1936	254	5.76%	4,141	93.94%	13	0.29%
1932	245	4.27%	5,429	94.61%	64	1.12%
1928	1,612	33.95%	3,128	65.88%	8	0.17%
1924	450	16.84%	1,967	73.59%	256	9.58%
1920	493	21.29%	1,763	76.12%	60	2.59%
1916	273	13.90%	1,691	86.10%	0	0.00%
1912	152	10.25%	1,090	73.50%	241	16.25%
1908	535	27.23%	1,407	71.60%	23	1.17%
1904	297	23.57%	955	75.79%	8	0.63%
1900	336	21.05%	1,238	77.57%	22	1.38%
1896	148	7.56%	1,749	89.37%	60	3.07%

• Future Interstate 69
• U.S. Highway 63
• U.S. Highway 82
• U.S. Highway 167
• Highway 7
• Highway 15
• Highway 129

• South Arkansas Regional Airport at Goodwin Field

• Calion
• El Dorado (county seat)
• Strong
• Huttig
• Junction City
• Norphlet
• Smackover

• Felsenthal

• Lawson
• Mount Holly
• Urbana

• Lapile
• Moro Bay
• Old Union
• New London
• Parkers Chapel

Townships in Arkansas are the divisions of a county. Each township includes unincorporated areas; some may have incorporated cities or towns within part of their boundaries. Arkansas townships have limited purposes in modern times. However, the United States census does list Arkansas population based on townships (sometimes referred to as "county subdivisions" or "minor civil divisions"). Townships are also of value for historical purposes in terms of genealogical research. Each town or city is within one or more townships in an Arkansas county based on census maps and publications. The townships of Union County are listed below; listed in parentheses are the cities, towns, and/or census-designated places that are fully or partially inside the township.

• List of lakes in Union County, Arkansas
• National Register of Historic Places listings in Union County, Arkansas

• Union County, Arkansas official site
• Union County, Arkansas entry on the Encyclopedia of Arkansas History & Culture

33°11′14″N 92°35′29″W / 33.18722°N 92.59139°W / 33.18722; -92.59139