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County · code 30043

Jefferson CountyMontana

Jefferson County is a county in Montana with its seat in Boulder. It was formed on January 16, 1864, and named after Thomas Jefferson.

  • Founded1864
  • County seatBoulder
  • Named forThomas Jefferson
  • Land area1,657 sq mi + 2.4 sq mi water
  • Population12,085 2020 Census
  • Parcels mapped13,278 checked 14 October 2026
Open the free map Click any parcel in Jefferson County: acreage, parcel id, address and who publishes the map. No sign-in.

Updated

Silver BowBroadwaterMadisonDeer LodgeLewis and ClarkPowellGallatin Boulder Census outline · seat marked

Where it is

Jefferson on the map of Montana

Montana has 56 counties and county equivalents, each with its own records office. Shapes with a page are links.

Beaverhead CountyBig Horn CountyBlaine CountyBroadwater CountyCarbon CountyCarter CountyCascade CountyChouteau CountyCuster CountyDaniels CountyDawson CountyDeer Lodge CountyFallon CountyFergus CountyFlathead CountyGallatin CountyGarfield CountyGlacier CountyGolden Valley CountyGranite CountyHill CountyJudith Basin CountyLake CountyLewis and Clark CountyLiberty CountyLincoln CountyMcCone CountyMadison CountyMeagher CountyMineral CountyMissoula CountyMusselshell CountyPark CountyPetroleum CountyPhillips CountyPondera CountyPowder River CountyPowell CountyPrairie CountyRavalli CountyRichland CountyRoosevelt CountyRosebud CountySanders CountySheridan CountySilver Bow CountyStillwater CountySweet Grass CountyTeton CountyToole CountyTreasure CountyValley CountyWheatland CountyWibaux CountyYellowstone CountyJefferson County Jefferson
Counties of Montana · Jefferson highlighted

History

Founded in 1864

An old deed may name a county that no longer exists, or a boundary that has since moved.

Jefferson County was created by Idaho Territory on January 16, 1864, and became a county in Montana Territory on May 26, 1864. The county was re-created by Montana Territory on February 2, 1865, after losing territory to the creation of Edgerton (now Lewis and Clark) and Gallatin counties. Over the following decades, Jefferson County exchanged and adjusted its boundaries with neighboring counties including Deer Lodge, Gallatin, Silver Bow, Powell, and Broadwater. It became a county in the state of Montana on November 8, 1889.

Boundary changes from the Newberry Library, Atlas of Historical County Boundaries; the 5 that matter most are shown first.

  1. 1864
    Jefferson County created Jefferson created by Idaho Territory from non-county area. · Idaho Terr. Laws 1863, 1st sess., sec. 5/pp. 675-676
  2. 1864
    Became part of Montana Jefferson became a county in Montana Territory. On 22 September 1864, Montana Territorial Governor, Sidney Edgerton, recognized the counties created by Idaho Territory in an election proclamation. · U.S. Stat., vol. 13, ch. 95 [1864], sec. 1/pp. 85-92; Sanders, Helen F., 1:329; Van Zandt, 145
  3. 1865
    Jefferson County created Jefferson re-created by Montana Territory; Jefferson lost to creation of Edgerton (now Lewis and Clark) and Gallatin, and lost to Deer Lodge and Beaverhead.
    from Edgertonfrom Lewis and Clarkfrom Gallatinfrom Deer Lodgefrom Beaverhead
    · Mont. Terr. Laws 1864, 1st leg., sec. 5/p. 530
  4. 1867
    Exchanged territory Jefferson exchanged with Edgerton (now Lewis and Clark).
    with Edgertonwith Lewis and Clark
    · Mont. Terr. Laws 1867, 4th leg., p. 101
  5. 1889
    Became part of Montana Jefferson became a county in the state of Montana. · U.S. Stat., vol. 26, pp. 1551-1552 and vol. 25, ch. 180 [1889]/pp. 676-684; Swindler, 6:82-83
Show all 13 changes (8 more)
  1. 1864
    Jefferson County created Jefferson created by Idaho Territory from non-county area. · Idaho Terr. Laws 1863, 1st sess., sec. 5/pp. 675-676
  2. 1864
    Became part of Montana Jefferson became a county in Montana Territory. On 22 September 1864, Montana Territorial Governor, Sidney Edgerton, recognized the counties created by Idaho Territory in an election proclamation. · U.S. Stat., vol. 13, ch. 95 [1864], sec. 1/pp. 85-92; Sanders, Helen F., 1:329; Van Zandt, 145
  3. 1865
    Jefferson County created Jefferson re-created by Montana Territory; Jefferson lost to creation of Edgerton (now Lewis and Clark) and Gallatin, and lost to Deer Lodge and Beaverhead.
    from Edgertonfrom Lewis and Clarkfrom Gallatinfrom Deer Lodgefrom Beaverhead
    · Mont. Terr. Laws 1864, 1st leg., sec. 5/p. 530
  4. 1866
    Boundary clarified Jefferson boundaries clarified (no change). Acts of the 2d and 3d legislatures were declared null and void by the United States Congress, 2 March 1867.
    no change
    · Mont. Terr. Laws 1866, 2d leg., ch. 19/p. 24; U.S. Stat., vol. 14, ch. 150 [1867], sec. 6/p. 427; Burlingame and Toole, 1:219-228; Sanders, Helen F., 1:331-341
  5. 1867
    Exchanged territory Jefferson exchanged with Edgerton (now Lewis and Clark).
    with Edgertonwith Lewis and Clark
    · Mont. Terr. Laws 1867, 4th leg., p. 101
  6. 1867
    Gained territory Jefferson gained from Deer Lodge, lost to Gallatin.
    from Deer Lodgefrom Gallatin
    · Mont. Terr. Laws 1867, 4th leg., pp. 104-105
  7. 1872
    Boundary clarified Jefferson boundaries redefined (no change).
    no change
    · Mont. Terr. Laws 1871, 7th leg., ch. 20, sec. 5/p. 430
  8. 1883
    Exchanged territory Jefferson exchanged with Silver Bow.
    with Silver Bow.
    · Mont. Terr. Laws 1883, 13th leg., p. 97
  9. 1887
    Boundary clarified Jefferson boundaries redefined (no change).
    no change
    · Mont. Terr. Laws 1887, 15th leg., secs. 734, 741/pp. 835, 838-839
  10. 1889
    Became part of Montana Jefferson became a county in the state of Montana. · U.S. Stat., vol. 26, pp. 1551-1552 and vol. 25, ch. 180 [1889]/pp. 676-684; Swindler, 6:82-83
  11. 1891
    Exchanged territory Jefferson exchanged with Deer Lodge.
    with Deer Lodge
    · Mont. Laws 1891, 2d leg., pp. 224-225
  12. 1897
    Lost territory Jefferson lost to creation of Broadwater. Act passed 9 February 1897; took effect 1 March 1897.
    to Broadwater
    · Mont. Laws 1897, 5th leg., pp. 45-49
  13. 1903
    Exchanged territory Jefferson exchanged with Deer Lodge and exchanged small areas with Powell. Act passed 4 March 1903; took effect 15 June 1903.
    with Deer Lodgewith Powell
    · Mont. Laws 1903, 8th leg., ch. 62/pp. 127-130

Land and parcels

What the maps cover in Jefferson

Who publishes each layer, how good it is, and when we last checked.

Official
from the office that keeps the record
Good enough
a reliable statewide or national copy
Fallback
a guide only; check the official record
Missing
nothing public found yet

People and land

Jefferson by the Census

Population
12,085 2020 Census count
Housing units
5,375 2020 Census count
Land area
1,657 square miles
Water area
2.4 square miles
People per sq mi
7 population ÷ land area

Source: US Census Bureau, 2020 Census; US Census Bureau, 2020 Gazetteer Files. Not endorsed or certified by the Census Bureau.

Rules and offices

Jefferson County's official site

We have found the county's site but not its offices yet; start there.

County government

See your own lot on these maps

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Questions

Common questions about Jefferson County

When was Jefferson County founded?
Jefferson County was founded in 1864, and named after Thomas Jefferson.
What is the county seat of Jefferson County?
The county seat of Jefferson County is Boulder.
Is there an online parcel map for Jefferson County?
Yes. A public source publishes a parcel map of 13,278 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Jefferson County's boundary or name changed?
Jefferson County was created in 1864. Since then the Atlas of Historical County Boundaries records 9 later changes, chiefly involving Deer Lodge County, Edgerton County and Gallatin County.
Is my lot in a flood zone?
We do not have flood maps for Jefferson County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Nearby

Counties next door