Yankton County is the only county in the Yankton Micropolitan Statistical Area.
As of the census of 2000, there were 21,652 people, 8,187 households, and 5,403 families residing in the county. The population density was 42 people per square mile (16/km²). There were 8,840 housing units at an average density of 17 per square mile (7/km²). The racial makeup of the county is 95.10% White, 1.16% African American, 1.63% Native American, 0.42% Asian, 0.02% Pacific Islander, 0.74% from other races, and 0.91% from two or more races. 1.82% of the population are Hispanic or Latino of any race.
There were 8,187 households out of which 33.00% had children under the age of 18 living with them, 54.40% were married couples living together, 8.30% had a female householder with no husband present, and 34.00% were non-families. 29.30% of all households were made up of individuals and 12.00% had someone living alone who was 65 years of age or older. The average household size was 2.43 and the average family size was 3.03.
In the county, the population was spread out with 25.70% under the age of 18, 8.70% from 18 to 24, 29.00% from 25 to 44, 22.00% from 45 to 64, and 14.60% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 101.90 males. For every 100 females age 18 and over, there were 101.30 males.
The median income for a household in the county was $35,374, and the median income for a family was $43,600. Males had a median income of $29,010 versus $20,686 for females. The per capita income for the county was $17,312. About 6.60% of families and 9.60% of the population were below the poverty line, including 10.00% of those under age 18 and 9.00% of those age 65 or over.