Adjacent counties and rural municipalities

National protected areas

• Black Coulee National Wildlife Refuge
• Nez Perce National Historical Park (part)
• Upper Missouri River Breaks National Monument (part)

2020 census

As of the 2020 census, there were 7,044 people living in the county.^{[citation needed]}

2010 census

As of the 2010 census, there were 6,491 people, 2,357 households, and 1,604 families living in the county. The population density was 1.5 inhabitants per square mile (0.58 inhabitants/km²). There were 2,843 housing units at an average density of 0.7 units per square mile (0.27 units/km²). The racial makeup of the county was 49.4% American Indian, 48.2% white, 0.1% black or African American, 0.1% Asian, 0.2% from other races, and 2.0% from two or more races. Those of Hispanic or Latino origin made up 1.8% of the population. In terms of ancestry, 23.0% were German, 9.2% were Norwegian, 6.6% were Irish, 5.7% were English, and 2.0% were American.

Of the 2,357 households, 39.2% had children under the age of 18 living with them, 45.6% were married couples living together, 16.0% had a female householder with no husband present, 31.9% were non-families, and 28.7% of all households were made up of individuals. The average household size was 2.66 and the average family size was 3.28. The median age was 35.1 years.

The median income for a household in the county was $37,034 and the median income for a family was $40,890. Males had a median income of $32,320 versus $28,986 for females. The per capita income for the county was $16,813. About 23.6% of families and 29.0% of the population were below the poverty line, including 44.6% of those under age 18 and 10.6% of those age 65 or over.

Cities

• Chinook (county seat)
• Harlem

Census-designated places

Unincorporated communities