Jefferson County, Montana

Jefferson County, Montana
Location within the U.S. state of Montana
Coordinates: 46°11′N 112°07′W / 46.18°N 112.11°W / 46.18; -112.11
Country United States
State Montana
FoundedFebruary 2, 1865
Named afterThomas Jefferson
SeatBoulder
Largest townClancy
Area
 • Total
1,659 sq mi (4,300 km2)
 • Land1,656 sq mi (4,290 km2)
 • Water2.6 sq mi (6.7 km2)  0.2%
Population
 (2020)
 • Total
12,085
 • Estimate 
(2024)
13,304
 • Density7/sq mi (2.7/km2)
Time zoneUTC−7 (Mountain)
 • Summer (DST)UTC−6 (MDT)
Congressional district2nd
Websitewww.jeffersoncounty-mt.gov
  • Montana county number 51

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.