Ziebach County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 2,413. Its county seat is Dupree. It is the last county (or county equivalent) in the United States alphabetically.

The county's per-capita income makes it the fourth-poorest county in the United States.

A county named Ziebach was created in Dakota Territory in 1877. However, after South Dakota became a state, this county was dissolved in 1898 and its areas absorbed by Pennington and Stanley counties. The present Ziebach County was created by the SD Legislature on February 1, 1911, and was fully organized by April 22. The 1911 Ziebach County encompassed parts of the former counties of Schnasse, Armstrong and Sterling, which were the last three extinct counties of South Dakota to cease to exist. It was named for Frank M. Ziebach, a political figure in the Dakota Territory during the territorial period from 1861 to 1889. Previously the area had been used by trappers and in 1907 part was briefly a reservation for Ute Indians displaced from Utah and Wyoming. Early in the 20th Century cattle were raised in substantial numbers, but when the railroad bypassed the area this industry declined. Limited homesteading also occurred on the more fertile lands.

The Cheyenne River flows east-northeastward along the southern boundary of Ziebach County. The Moreau River flows eastward through the upper portion of the county, and Cherry Creek flows southeastward through the lower portion, draining the area into the Cheyenne River. The terrain is composed of semi-arid rolling hills interrupted by buttes and carved by drainages and gullies, partly devoted to agriculture and cattle. The terrain slopes to the south and the east; its highest point (except for the isolated Thunder Butte, at 2,733') is near its NW corner at 2,582 ft (787 m) ASL. The county has a total area of 1,971 square miles (5,100 km), of which 1,961 square miles (5,080 km) is land and 9.3 square miles (24 km) (0.5%) is water. Almost the entire county lies within the Cheyenne River Indian Reservation. The balance of the county, along its extreme northern county line, lies within the Standing Rock Indian Reservation. It is one of five South Dakota counties that are comprised entirely of land that is within the boundaries of Indian reservations.

Ziebach County is part of the Great Plains and is characterized by rolling grasslands and numerous buttes. The buttes form the highest points of elevation in Ziebach County:

• Bednor Lake State Game Production Area
• Cheyenne State Game Production Area (part)
• Grand River National Grassland (part)

• Bedners Dam
• K C Dam
• Rattlesnake Lake

Historical population

Census	Pop.	Note	%±
1920	3,718		—
1930	4,039		8.6%
1940	2,875		−28.8%
1950	2,606		−9.4%
1960	2,495		−4.3%
1970	2,221		−11.0%
1980	2,308		3.9%
1990	2,220		−3.8%
2000	2,519		13.5%
2010	2,801		11.2%
2020	2,413		−13.9%
2023 (est.)	2,322		−3.8%
U.S. Decennial Census 1790-1960 1900-1990 1990-2000 2010-2020

As of the 2020 census, there were 2,413 people, 732 households, and 528 families in the county. The population density was 1.2 inhabitants per square mile (0.46/km). There were 882 housing units.

As of the 2010 census, there were 2,801 people, 836 households, and 638 families in the county. The population density was 1.4 inhabitants per square mile (0.54/km). There were 987 housing units at an average density of 0.5 per square mile (0.19/km). The racial makeup of the county was 74.9% American Indian, 21.8% white, 0.2% black or African American, 0.1% Asian, 0.1% from other races, and 3.0% from two or more races. Those of Hispanic or Latino origin made up 3.1% of the population.

Of the 836 households, 53.7% had children under the age of 18 living with them, 39.0% were married couples living together, 29.3% had a female householder with no husband present, 23.7% were non-families, and 20.9% of all households were made up of individuals. The average household size was 3.35 and the average family size was 3.82. The median age was 25.4 years.

The median income for a household in the county was $27,578 and the median income for a family was $22,857. Males had a median income of $28,954 versus $24,327 for females. The per capita income for the county was $11,069. About 41.9% of families and 46.0% of the population were below the poverty line, including 56.1% of those under age 18 and 22.9% of those age 65 or over.

• Dupree (county seat)
• Eagle Butte (partial)

• Bridger
• Cherry Creek

Ziebach County government does not include subdivision into townships. The county is divided into three areas of unorganized territory: Dupree, North Ziebach, and South Ziebach.

Ziebach County has traditionally been a swing county. Only Ronald Reagan in 1980 and Barack Obama in 2008 have topped sixty percent for either major party in the past six decades. Donald Trump won 48% of the vote in 2016, being the county or equivalent he won with the highest percentage of Native Americans. Joe Biden won 53% of the vote in 2020.

United States presidential election results for Ziebach County, South Dakota

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2020	404	44.59%	481	53.09%	21	2.32%
2016	368	47.98%	353	46.02%	46	6.00%
2012	314	41.26%	439	57.69%	8	1.05%
2008	312	35.02%	554	62.18%	25	2.81%
2004	447	40.05%	641	57.44%	28	2.51%
2000	384	53.26%	314	43.55%	23	3.19%
1996	375	40.32%	483	51.94%	72	7.74%
1992	328	44.93%	280	38.36%	122	16.71%
1988	362	45.53%	427	53.71%	6	0.75%
1984	429	54.17%	359	45.33%	4	0.51%
1980	523	65.05%	246	30.60%	35	4.35%
1976	369	48.49%	370	48.62%	22	2.89%
1972	486	55.86%	378	43.45%	6	0.69%
1968	449	52.58%	350	40.98%	55	6.44%
1964	447	44.66%	554	55.34%	0	0.00%
1960	568	52.59%	512	47.41%	0	0.00%
1956	627	57.05%	472	42.95%	0	0.00%
1952	779	67.68%	372	32.32%	0	0.00%
1948	463	47.24%	503	51.33%	14	1.43%
1944	331	48.32%	354	51.68%	0	0.00%
1940	691	50.15%	687	49.85%	0	0.00%
1936	561	42.53%	737	55.88%	21	1.59%
1932	462	31.36%	982	66.67%	29	1.97%
1928	759	55.00%	615	44.57%	6	0.43%
1924	659	55.01%	153	12.77%	386	32.22%
1920	507	65.42%	177	22.84%	91	11.74%
1916	275	54.89%	211	42.12%	15	2.99%
1912	0	0.00%	349	45.56%	417	54.44%

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

44°59′N 101°40′W / 44.98°N 101.67°W / 44.98; -101.67