2000 census

As of the 2020 census, there were 8,329 people living in the county.^{[citation needed]}

2010 census

As of the 2010 census, there were 9,233 people, 3,395 households, and 2,318 families residing in the county. The population density was 1.8 inhabitants per square mile (0.69/km²). There were 4,057 housing units at an average density of 0.8 per square mile (0.31/km²). The racial makeup of the county was 61.3% white, 34.7% American Indian, 0.5% Asian, 0.3% black or African American, 0.5% from other races, and 2.8% from two or more races. Those of Hispanic or Latino origin made up 3.4% of the population. In terms of ancestry, 25.0% were American, 19.0% were German, 6.9% were English, and 6.6% were Irish.

Of the 3,395 households, 36.4% had children under the age of 18 living with them, 51.3% were married couples living together, 10.6% had a female householder with no husband present, 31.7% were non-families, and 27.6% of all households were made up of individuals. The average household size was 2.70 and the average family size was 3.32. The median age was 36.5 years.

The median income for a household in the county was $44,776 and the median income for a family was $56,282. Males had a median income of $52,500 versus $28,306 for females. The per capita income for the county was $19,844. About 13.6% of families and 18.5% of the population were below the poverty line, including 24.7% of those under age 18 and 21.3% of those age 65 or over.

Cities

• Colstrip
• Forsyth (county seat)

Census-designated places

Unincorporated communities

Former communities

• Vananda