2020 census
As of the 2020 census, the county had a population of 19,956. [15] There were 8,581 households and 4,983 families residing in the county. [16] The population density was 45.8 inhabitants per square mile (17.7/km2).
Of the residents, 23.0% were under the age of 18 and 19.6% were 65 years of age or older; the median age was 39.5 years. For every 100 females there were 99.5 males, and for every 100 females age 18 and over there were 99.2 males. [15]
The racial makeup of the county was 89.1% White, 0.8% Black or African American, 3.3% American Indian and Alaska Native, 0.6% Asian, 2.2% from some other race, and 4.0% from two or more races. Hispanic or Latino residents of any race comprised 4.6% of the population. [17]
There were 8,581 households in the county, of which 26.6% had children under the age of 18 living with them and 26.0% had a female householder with no spouse or partner present. About 35.5% of all households were made up of individuals and 14.5% had someone living alone who was 65 years of age or older. [15]
There were 9,484 housing units, of which 9.5% were vacant. Among occupied housing units, 60.9% were owner-occupied and 39.1% were renter-occupied. The homeowner vacancy rate was 1.7% and the rental vacancy rate was 13.1%. [15]
2010 census
As of the 2010 census, there were 19,504 people, 8,296 households, and 4,892 families in the county. The population density was 44.8 inhabitants per square mile (17.3/km2). There were 8,852 housing units at an average density of 20.3 per square mile (7.8/km2). The racial makeup of the county was 94.4% white, 2.5% American Indian, 0.5% Asian, 0.4% black or African American, 0.1% Pacific islander, 0.5% from other races, and 1.6% from two or more races. Those of Hispanic or Latino origin made up 1.5% of the population. In terms of ancestry, 51.8% were German, 10.6% were Norwegian, 10.1% were Irish, 7.4% were English, 6.0% were Dutch, and 4.5% were American.
Of the 8,296 households, 28.1% had children under the age of 18 living with them, 46.6% were married couples living together, 8.5% had a female householder with no husband present, 41.0% were non-families, and 34.3% of all households were made up of individuals. The average household size was 2.26 and the average family size was 2.91. The median age was 37.8 years.
The median income for a household in the county was $41,867 and the median income for a family was $54,677. Males had a median income of $37,688 versus $26,223 for females. The per capita income for the county was $22,794. About 6.9% of families and 13.8% of the population were below the poverty line, including 12.1% of those under age 18 and 10.4% of those age 65 or over.
United States
South Dakota 
