| County | FIPS Code | County Seat | Established in | Population | Area (sq mi) | Area (km2) |
|---|---|---|---|---|---|---|
| Beaverhead County | 1 | Dillon | February 2, 1865 | 9,345 | 5,543 | 14,356 |
| Big Horn County | 3 | Hardin | January 13,1913 | 13,282 | 4,995 | 12,937 |
| Blaine County | 5 | Chinook | February 29, 1912 | 6,619 | 4,226 | 10,945 |
| Broadwater County | 7 | Townsend | February 9, 1897 | 5,667 | 1,192 | 3,087 |
| Carbon County | 9 | Red Lodge | March 4, 1895 | 10,399 | 2,048 | 5,304 |
| Carter County | 11 | Ekalaka | February 22, 1917 | 1,169 | 3,340 | 8,651 |
| Cascade County | 13 | Great Falls | September 12, 1887 | 81,366 | 2,698 | 6,988 |
| Chouteau County | 15 | Fort Benton | February 2, 1865 | 5,894 | 3,973 | 10,290 |
| Custer County | 17 | Miles City | February 2, 1865 | 12,092 | 3,783 | 9,798 |
| Daniels County | 19 | Scobey | August 30, 1920 | 1,793 | 1,426 | 3,693 |
| Dawson County | 21 | Glendive | January 15, 1869 | 9,518 | 2,373 | 6,146 |
| Deer Lodge County | 23 | Anaconda | February 2, 1865 | 9,150 | 737 | 1,909 |
| Fallon County | 25 | Baker | December 9, 1913 | 3,108 | 1,620 | 4,196 |
| Fergus County | 27 | Lewistown | March 12, 1885 | 11,442 | 4,339 | 11,238 |
| Flathead County | 29 | Kalispell | February 6, 1893 | 103,806 | 5,099 | 13,206 |
| Gallatin County | 31 | Bozeman | February 2, 1865 | 114,434 | 2,507 | 6,493 |
| Garfield County | 33 | Jordan | February 7, 1919 | 1,309 | 4,668 | 12,090 |
| Glacier County | 35 | Cut Bank | February 17, 1919 | 13,696 | 2,995 | 7,757 |
| Golden Valley County | 37 | Ryegate | October 4, 1920 | 852 | 1,175 | 3,043 |
| Granite County | 39 | Philipsburg | March 2, 1893 | 3,209 | 1,728 | 4,475 |
| Hill County | 41 | Havre | February 22, 1912 | 16,596 | 2,896 | 7,501 |
| Jefferson County | 43 | Boulder | February 2, 1865 | 11,558 | 1,657 | 4,292 |
| Judith Basin County | 45 | Stanford | December 10, 1920 | 1,991 | 1,870 | 4,843 |
| Lake County | 47 | Polson | May 11, 1923 | 29,099 | 1,494 | 3,869 |
| Lewis and Clark County | 49 | Helena | February 2, 1865 | 69,432 | 3,461 | 8,964 |
| Liberty County | 51 | Chester | February 11, 1920 | 2,359 | 1,430 | 3,704 |
| Lincoln County | 53 | Libby | March 9, 1909 | 19,125 | 3,613 | 9,358 |
| McCone County | 55 | Circle | February 20, 1919 | 1,694 | 2,643 | 6,845 |
| Madison County | 57 | Virginia City | February 2, 1865 | 7,820 | 3,587 | 9,290 |
| Meagher County | 59 | White Sulphur Springs | November 16, 1867 | 1,853 | 2,392 | 6,195 |
| Mineral County | 61 | Superior | August 7, 1914 | 4,316 | 1,220 | 3,160 |
| Missoula County | 63 | Missoula | February 2, 1865 | 119,600 | 2,598 | 6,729 |
| Musselshell County | 65 | Roundup | February 11, 1911 | 4,651 | 1,867 | 4,836 |
| Park County | 67 | Livingston | February 23, 1887 | 16,606 | 2,656 | 6,879 |
| Petroleum County | 69 | Winnett | November 24, 1924 | 513 | 1,654 | 4,284 |
| Phillips County | 71 | Malta | February 5, 1915 | 4,074 | 5,140 | 13,313 |
| Pondera County | 73 | Conrad | February 17, 1919 | 5,972 | 1,625 | 4,209 |
| Powder River County | 75 | Broadus | March 7, 1919 | 1,716 | 3,297 | 8,539 |
| Powell County | 77 | Deer Lodge | January 31, 1901 | 6,968 | 2,326 | 6,024 |
| Prairie County | 79 | Terry | February 5, 1915 | 1,087 | 1,737 | 4,499 |
| Ravalli County | 81 | Hamilton | February 16, 1893 | 43,806 | 2,394 | 6,200 |
| Richland County | 83 | Sidney | May 27, 1914 | 10,913 | 2,084 | 5,398 |
| Roosevelt County | 85 | Wolf Point | February 18, 1919 | 11,059 | 2,356 | 6,102 |
| Rosebud County | 87 | Forsyth | February 11, 1901 | 9,063 | 5,012 | 12,981 |
| Sanders County | 89 | Thompson Falls | February 7, 1905 | 11,844 | 2,762 | 7,154 |
| Sheridan County | 91 | Plentywood | March 24, 1913 | 3,424 | 1,677 | 4,343 |
| Silver Bow County | 93 | Butte | February 16, 1881 | 34,915 | 718 | 1,860 |
| Stillwater County | 95 | Columbus | March 24, 1913 | 9,534 | 1,795 | 4,649 |
| Sweet Grass County | 97 | Big Timber | March 5, 1895 | 3,710 | 1,855 | 4,804 |
| Teton County | 99 | Choteau | February 7, 1893 | 6,162 | 2,273 | 5,887 |
| Toole County | 101 | Shelby | May 7, 1914 | 4,853 | 1,911 | 4,949 |
| Treasure County | 103 | Hysham | February 7, 1919 | 679 | 979 | 2,536 |
| Valley County | 105 | Glasgow | February 6, 1893 | 7,437 | 4,921 | 12,745 |
| Wheatland County | 107 | Harlowton | February 22, 1917 | 2,236 | 1,423 | 3,686 |
| Wibaux County | 109 | Wibaux | August 17, 1914 | 1,034 | 889 | 2,302 |
| Yellowstone County | 111 | Billings | February 26, 1883 | 161,300 | 2,635 | 6,825 |
