Jefferson County is a county in Montana, United States. As of the 2020 census, the population was 12,085. Its county seat is Boulder. The county was created in 1865 and named for President Thomas Jefferson.
Jefferson County is part of the Helena, MT Micropolitan Statistical Area.
Geography
According to the United States Census Bureau, the county has a total area of , of which is land and (0.2%) is water.
Major highways
Interstate 15
Interstate 90
U.S. Highway 12
U.S. Highway 287
Montana Highway 2
Montana Highway 41
Montana Highway 55
Montana Highway 69
Adjacent counties
Lewis and Clark County - north
Broadwater County - east
Gallatin County - southeast
Madison County - south
Silver Bow County - west
Deer Lodge County - west
Powell County - northwest
National protected areas
Deerlodge National Forest (part) Helena National Forest (part)
Politics
Demographics
2020 census
As of the census of 2020, there were 12,085 people.
2010 census
As of the census of 2010, there were 11,406 people, 4,512 households, and 3,301 families residing in the county. The population density was . There were 5,055 housing units at an average density of . The racial makeup of the county was 95.5% white, 1.4% American Indian, 0.4% Asian, 0.1% black or African American, 0.4% from other races, and 2.2% from two or more races. Those of Hispanic or Latino origin made up 2.0% of the population. In terms of ancestry, 31.3% were German, 19.1% were Irish, 17.2% were English, 10.8% were Norwegian, and 4.2% were American.
Of the 4,512 households, 28.8% had children under the age of 18 living with them, 63.7% were married couples living together, 5.4% had a female householder with no husband present, 26.8% were non-families, and 22.6% of all households were made up of individuals. The average household size was 2.48 and the average family size was 2.90. The median age was 46.2 years.
The median income for a household in the county was $56,695 and the median income for a family was $67,195. Males had a median income of $50,978 versus $34,148 for females. The per capita income for the county was $26,437. About 2.7% of families and 12.8% of the population were below the poverty line, including 5.1% of those under age 18 and 8.6% of those age 65 or over.
Communities
Towns
Boulder (county seat) Whitehall
Census-designated places
Basin
Cardwell
Clancy (includes Alhambra) Elkhorn Jefferson City Montana City Rader Creek South Hills