Custer County lies on the west line of South Dakota. Its west boundary line abuts the east boundary line of the state of Wyoming. The Cheyenne River flows northeastward along the upper portion of the county's east boundary. Battle Creek flows southeastward in the upper eastern part of the county, discharging into Cheyenne River along the county's northeastern boundary line. Spring Creek flows northeastward through the upper eastern part of the county, discharging into the river just north of the county border. The county terrain is mountainous, especially its western portion. The terrain slopes to the east; its lowest point is its NE corner at 2,461' above sea level. Its highest point is a mountain crest along the north boundary line, at 6,657' ASL. Custer County has a total area of, of which is land and is water.
As of the 2000 United States Census, there were 7,275 people, 2,970 households, and 2,067 families in the county. The population density was 5 people per square mile. There were 3,624 housing units at an average density of 2 per square mile. The racial makeup of the county was 94.17% White, 0.27% Black or African American, 3.12% Native American, 0.18% Asian, 0.01% Pacific Islander, 0.36% from other races, and 1.88% from two or more races. 1.51% of the population were Hispanic or Latino of any race. 32.2% were of German, 9.8% Irish, 9.2% English, 7.1% Norwegian and 5.7% American ancestry. There were 2,970 households out of which 26.90% had children under the age of 18 living with them, 60.20% were married couples living together, 6.60% had a female householder with no husband present, and 30.40% were non-families. 25.90% of all households were made up of individuals and 10.30% had someone living alone who was 65 years of age or older. The average household size was 2.35 and the average family size was 2.80. The county population contained 24.10% under the age of 18, 6.30% from 18 to 24, 22.40% from 25 to 44, 31.10% from 45 to 64, and 16.00% who were 65 years of age or older. The median age was 43 years. For every 100 females there were 104.40 males. For every 100 females age 18 and over, there were 99.40 males. The median income for a household in the county was $36,303, and the median income for a family was $43,628. Males had a median income of $30,475 versus $20,781 for females. The per capita income for the county was $17,945. About 6.20% of families and 9.40% of the population were below the poverty line, including 13.00% of those under age 18 and 7.40% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 8,216 people, 3,636 households, and 2,427 families in the county. The population density was. There were 4,628 housing units at an average density of. The racial makeup of the county was 94.2% white, 2.9% American Indian, 0.4% Asian, 0.2% black or African American, 0.4% from other races, and 2.0% from two or more races. Those of Hispanic or Latino origin made up 2.2% of the population. In terms of ancestry, 42.1% were German, 13.1% were Irish, 11.4% were English, 10.8% were Norwegian, and 7.9% were American. Of the 3,636 households, 21.7% had children under the age of 18 living with them, 58.2% were married couples living together, 5.5% had a female householder with no husband present, 33.3% were non-families, and 28.4% of all households were made up of individuals. The average household size was 2.19 and the average family size was 2.65. The median age was 50.3 years. The median income for a household in the county was $46,743 and the median income for a family was $58,253. Males had a median income of $39,194 versus $29,375 for females. The per capita income for the county was $24,353. About 4.3% of families and 9.7% of the population were below the poverty line, including 10.9% of those under age 18 and 11.7% of those age 65 or over.
