Founded in February 2, 1865, Jefferson County is one of 56 counties in the state of Montana. Historically, Jefferson County was one of the 9 original counties. The city of Boulder is the county seat. With FIPS code of 43, Jefferson County has a population of 12,470 and area of 1,657 square miles (4,292 km2).
List of all cities in Jefferson County, MT alphabetically
| City/Town | Postal Code | Area Code |
| Basin, MT | 59631 | 406 |
| Boulder, MT | 59632 | 406 |
| Cardwell, MT | 59721 | 406 |
| Clancy, MT | 59634 | 406 |
| Jefferson City, MT | 59638 | 406 |
| Montana City, MT | 59634 | 406 |
| Whitehall, MT | 59759 | 406 |