Sully County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 1,446, making it the fifth-least populous county in South Dakota. Its county seat is Onida. The county was created in 1873 and organized in 1883. It is named after General Alfred Sully, who built Fort Sully.

Sully County is included in the Pierre, SD Micropolitan Statistical Area.

Geography

The west boundary line of Sully County is defined by the meanderings of the Missouri River, which flows southward along its edge. The county's terrain is composed of semi-arid rolling hills, partially devoted to agriculture. The terrain slopes to the south and east, but the west portion of the county slopes westward into the river valley. The county's highest point is along the midpoint of its north boundary line, at 1,949 ft (594 m) ASL. The county has a total area of 1,070 square miles (2,800 km), of which 1,007 square miles (2,610 km) is land and 63 square miles (160 km) (5.9%) is water.

The eastern portion of South Dakota's counties (48 of 66) observe Central Time; the western counties (18 of 66) observe Mountain Time. Sully County is at the western edge of those counties that observe Central Time.

Major highways

• U.S. Highway 83
• South Dakota Highway 1804

Adjacent counties

Protected areas

Lakes

Demographics

Historical population

Census	Pop.	Note	%±
1880	296		—
1890	2,412		714.9%
1900	1,715		−28.9%
1910	2,462		43.6%
1920	2,831		15.0%
1930	3,852		36.1%
1940	2,668		−30.7%
1950	2,713		1.7%
1960	2,607		−3.9%
1970	2,362		−9.4%
1980	1,990		−15.7%
1990	1,589		−20.2%
2000	1,556		−2.1%
2010	1,373		−11.8%
2020	1,446		5.3%
2023 (est.)	1,494		3.3%
U.S. Decennial Census 1790–1960 1900–1990 1990–2000 2010–2020

2020 census

As of the 2020 census, there were 1,446 people, 635 households, and 428 families residing in the county. The population density was 1.4 inhabitants per square mile (0.54/km). There were 892 housing units.

2010 census

As of the 2010 census, there were 1,373 people, 610 households, and 397 families in the county. The population density was 1.4 inhabitants per square mile (0.54/km). There were 845 housing units at an average density of 0.84 units per square mile (0.32 units/km). The racial makeup of the county was 96.6% white, 1.2% American Indian, 0.1% black or African American, 0.0% from other races, and 2.2% from two or more races. Those of Hispanic or Latino origin made up 0.9% of the population. In terms of ancestry,

Of the 610 households, 27.0% had children under the age of 18 living with them, 57.5% were married couples living together, 4.6% had a female householder with no husband present, 34.9% were non-families, and 31.6% of all households were made up of individuals. The average household size was 2.25 and the average family size was 2.82. The median age was 46.6 years.

The median income for a household in the county was $48,958 and the median income for a family was $58,875. Males had a median income of $34,375 versus $29,087 for females. The per capita income for the county was $26,596. About 4.4% of families and 8.0% of the population were below the poverty line, including 12.8% of those under age 18 and 8.4% of those age 65 or over.

Communities

City

• Onida (county seat)

Town

• Agar

Census-designated place

• Cow Creek

Unorganized territories

The county organization does not include division into townships. Its area is divided into two areas of unorganized territory: West Sully and East Sully.

Politics

Throughout its history, Sully County has been powerfully Republican. The solitary Democrat to carry Sully County at a Presidential level has been Franklin D. Roosevelt in 1932 during an election heavily influenced by the "Dust Bowl" and Great Depression. Nonetheless, in the following 1936 election, Alf Landon won the county by over twenty percent. Since that time, the Democratic Party has bettered FDR's 1936 effort five times, but only Lyndon Johnson in 1964 has held the GOP to a single-figure margin. In modern times, like almost all of rural America, Sully County has become more and more Republican. The last Democrat to carry one-third of the county's vote was Michael Dukakis in 1988 during an election severely affected by a major drought.

United States presidential election results for Sully County, South Dakota

Year	Republican		Democratic		Third party
	No.	%	No.	%	No.	%
2020	726	78.06%	185	19.89%	19	2.04%
2016	679	78.86%	137	15.91%	45	5.23%
2012	613	74.94%	186	22.74%	19	2.32%
2008	581	69.75%	233	27.97%	19	2.28%
2004	702	76.55%	201	21.92%	14	1.53%
2000	633	72.68%	209	24.00%	29	3.33%
1996	592	57.76%	321	31.32%	112	10.93%
1992	565	55.94%	273	27.03%	172	17.03%
1988	571	56.76%	393	39.07%	42	4.17%
1984	836	75.52%	266	24.03%	5	0.45%
1980	852	74.15%	220	19.15%	77	6.70%
1976	630	55.31%	505	44.34%	4	0.35%
1972	773	64.90%	414	34.76%	4	0.34%
1968	676	60.09%	356	31.64%	93	8.27%
1964	667	52.81%	596	47.19%	0	0.00%
1960	864	64.14%	483	35.86%	0	0.00%
1956	726	59.51%	494	40.49%	0	0.00%
1952	860	70.96%	352	29.04%	0	0.00%
1948	579	58.37%	405	40.83%	8	0.81%
1944	612	67.11%	300	32.89%	0	0.00%
1940	840	64.71%	458	35.29%	0	0.00%
1936	667	58.61%	437	38.40%	34	2.99%
1932	559	35.63%	961	61.25%	49	3.12%
1928	999	70.50%	415	29.29%	3	0.21%
1924	555	59.04%	138	14.68%	247	26.28%
1920	542	62.95%	147	17.07%	172	19.98%
1916	281	50.45%	268	48.11%	8	1.44%
1912	0	0.00%	242	42.31%	330	57.69%
1908	368	68.27%	154	28.57%	17	3.15%
1904	364	82.92%	50	11.39%	25	5.69%
1900	294	64.76%	152	33.48%	8	1.76%
1896	262	56.34%	198	42.58%	5	1.08%
1892	278	57.44%	39	8.06%	167	34.50%

Notable person

• Irwin Gunsalus (1912–2008), biochemist, was born in Sully County.

See also

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