Jefferson County is a county in Idaho, United States, in the upper Snake River Plain northeast of Idaho Falls. It had a population of 30,891 at the 2020 census and covers 1,106 square miles (2,860 km2), of which 1,094 square miles (2,830 km2) is land, centred near 43°49′N 112°19′W.
Rigby is the county seat, and its postal code is 83442. The county has six incorporated cities and towns plus several unincorporated communities; it does not use civil townships. Jefferson County sits in the Mountain time zone and carries FIPS code 16051.
Towns & Villages in Jefferson County
Jefferson County has six incorporated municipalities and a handful of unincorporated crossroads communities scattered across its farmland. CDP = census-designated place; Unincorp. = unincorporated community with no formal municipal government.
| Place | Type | Pop. | ZIP |
|---|---|---|---|
| Rigby | City · seat | 5,038 | 83442 |
| Ririe | City | 667 | 83443 |
| Menan | City | 715 | 83434 |
| Roberts | City | 500 | 83444 |
| Lewisville | City | 421 | 83431 |
| Mud Lake | City | 321 | — |
| Terreton | Unincorp. | — | 83450 |
| Hamer | Unincorp. | — | 83425 |
| Monteview | Unincorp. | — | 83435 |
| Annis | Unincorp. | — | — |
| Camas | Unincorp. | — | — |
| Garfield | Unincorp. | — | — |
| Grant | Unincorp. | — | — |
| Heise | Unincorp. | — | — |
| Labelle | Unincorp. | — | — |
| Lorenzo | Unincorp. | — | — |
| Midway | Unincorp. | — | — |
| Rudy | Unincorp. | — | — |
| Sage Junction | Unincorp. | — | — |
Rigby accounts for roughly three-quarters of the county’s population and is the retail and school-district hub; Ririe, Menan, Roberts, Lewisville and Mud Lake are each small farming towns with their own post offices. The unincorporated places listed above are rural crossroads or former rail sidings without their own municipal government, several sharing a ZIP code with a nearby incorporated town.
ZIP Codes
- 83442 — Rigby
- 83434 — Menan
- 83443 — Ririe
- 83444 — Roberts
- 83431 — Lewisville
- 83450 — Terreton (also serves Mud Lake)
- 83425 — Hamer
- 83435 — Monteview
Smaller crossroads communities such as Annis, Camas, Garfield, Grant, Heise, Labelle, Lorenzo, Midway, Rudy and Sage Junction have no ZIP code of their own and are served by the nearest listed post-office town.
Geography
Jefferson County lies on the flat, irrigated upper Snake River Plain, with the Snake River forming part of its southern edge and the South Fork of the Teton River crossing the county’s east side. Sagebrush steppe and lava-rock terrain cover the county’s northwestern reaches around Mud Lake and the Idaho National Laboratory boundary; irrigated farmland dominates the central and eastern portions.
Jefferson County borders Fremont County to the northeast, Madison County to the east, Bonneville County to the south, Bingham County to the southwest, and Butte County to the west. Clark County lies to the north.
Demographics
Jefferson County had 30,891 residents at the 2020 census. Rigby, with 5,038 residents, and Menan, with 715, are the county’s two largest communities. Median household income in the county runs below the Idaho median of $74,942, reflecting its largely agricultural economy. Population density is roughly 28 people per square mile.
Economy & Local Life
Agriculture anchors Jefferson County’s economy, with potatoes, wheat, barley and hay grown on irrigated fields fed by the Snake River aquifer; dairy and cattle operations are also common. Rigby serves as the county’s commercial and school-district center, home to Jefferson School District 251 and Rigby High School. Proximity to the Idaho National Laboratory, headquartered in neighboring Butte County, provides research and technical employment for county residents as well.
History
Jefferson County was organized in 1913 out of Fremont County and named for Thomas Jefferson, the third U.S. president. The area’s first post office opened at Market Lake, a settlement near present-day Roberts, in 1868, predating the county itself by more than four decades. Irrigation canals built in the early 1900s converted the sagebrush plain into farmland and drew the homesteaders who founded Rigby and its neighboring towns.
Getting There
Interstate 15 runs north–south through the county, with US Route 20 crossing it and Idaho State Highways 22, 28 and 48 serving local towns.
- Idaho Falls — 15 miles southeast
- Rexburg — 12 miles northeast
- Blackfoot — 30 miles south
- Idaho National Laboratory site — 25 miles west
The nearest commercial airport is Idaho Falls Regional Airport, about 15 miles southeast of Rigby.
Frequently Asked Questions
Where is Jefferson County, Idaho?
It sits on the upper Snake River Plain in eastern Idaho, northeast of Idaho Falls and west of Rexburg.
What is the population of Jefferson County?
30,891, as of the 2020 census.
How big is Jefferson County?
1,106 square miles (2,860 km2), of which 1,094 square miles is land.
What is the county seat of Jefferson County?
Rigby is the county seat.
How many towns are in Jefferson County?
Six incorporated cities and towns — Rigby, Ririe, Menan, Roberts, Lewisville and Mud Lake — plus several unincorporated communities.
What ZIP codes are in Jefferson County?
83442 (Rigby), 83434 (Menan), 83443 (Ririe), 83444 (Roberts), 83431 (Lewisville), 83450 (Terreton), 83425 (Hamer) and 83435 (Monteview).
What is Jefferson County known for?
Irrigated potato and grain farming, and its ties to the nearby Idaho National Laboratory research site.
More counties in this state: Towns and villages in Idaho.