Dewey County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 5,239. Its county seat is Timber Lake. The county was created in 1883 and organized in 1910. It was named for William P. Dewey, Territorial surveyor-general from 1873 to 1877.
Almost the entire county lies in the Cheyenne River Indian Reservation. The balance of the county, along its extreme northern county line, lies in the Standing Rock Sioux Reservation. It is one of five South Dakota counties that are contained within Indian reservations.
The Moreau River flows east-northeasterly through the upper central parts of Dewey County, discharging into the Missouri River near the county's NE corner. Smaller drainages move runoff water northward from the central-eastern portions to the Missouri River, discharging near the community of Promise. A significant arm of the Missouri River forms the county's southeastern border. The county terrain consists of rolling hills, sloping southeastward and dropping off into the Missouri River basin.
The county has a total area of , of which is land and (5.8%) 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. Dewey County is the easternmost of the SD counties to observe Mountain Time.
As of the 2020 census, the county had a population of 5,239, 1,705 households, and 1,180 families, yielding a population density of .
Of the residents, 31.4% were under the age of 18 and 12.0% were 65 years of age or older; the median age was 32.4 years. For every 100 females there were 100.0 males, and for every 100 females age 18 and over there were 97.6 males.
The racial makeup of the county was 16.3% White, 0.2% Black or African American, 78.9% American Indian and Alaska Native, 0.1% Asian, 0.3% from some other race, and 4.2% from two or more races. Hispanic or Latino residents of any race comprised 1.5% of the population.
There were 1,705 households in the county, of which 42.9% had children under the age of 18 living with them and 31.8% had a female householder with no spouse or partner present. About 24.9% of all households were made up of individuals and 8.6% had someone living alone who was 65 years of age or older.
There were 1,923 housing units, of which 11.3% were vacant. Among occupied housing units, 58.4% were owner-occupied and 41.6% were renter-occupied. The homeowner vacancy rate was 0.3% and the rental vacancy rate was 10.3%.
As of the 2010 census, there were 5,301 people, 1,730 households, and 1,239 families in the county. The population density was . There were 2,002 housing units at an average density of . The racial makeup of the county was 74.9% Native American, 21.0% white, 0.2% Asian, 0.1% black or African American, 0.2% from other races, and 3.6% from two or more races. Those of Hispanic or Latino origin made up 1.8% of the population. In terms of ancestry, 13.2% were German, 6.5% were Irish, and 0.6% were American.
Of the 1,730 households, 45.9% had children under the age of 18 living with them, 37.6% were married couples living together, 24.2% had a female householder with no husband present, 28.4% were non-families, and 24.6% of all households were made up of individuals. The average household size was 3.05 and the average family size was 3.60. The median age was 30.0 years.
The median income for a household in the county was $33,255 and the median income for a family was $40,500. Males had a median income of $33,942 versus $28,594 for females. The per capita income for the county was $15,632. About 20.5% of families and 30.5% of the population were below the poverty line, including 39.5% of those under age 18 and 13.5% of those age 65 or over.
The county is divided into two areas of unorganized territory: North Dewey and South Dewey.
Dewey has since the 1990s been a strongly Democratic county in solidly Republican South Dakota. The last Republican to carry the county was Ronald Reagan in his 1984 landslide when he came within 3,761 votes of claiming all fifty states. Before this period, by contrast, Dewey was a Republican-leaning county even for South Dakota. Between its formation and 1984, Dewey had voted Democratic only in the three landslide Democratic wins of 1964, 1936 and 1932, plus for Woodrow Wilson in 1916 when his anti-war policies had strong appeal in the West.
School districts include: