| County | FIPS Code | County Seat | Established | License Plate Designator | Population | Area (sq mi) | Area (km2) |
|---|---|---|---|---|---|---|---|
| Ada County | 1 | Boise | 1864 | 1A | 392,365 | 1,055 | 2,732 |
| Adams County | 3 | Council | 1911 | 2A | 3,976 | 1,365 | 3,535 |
| Bannock County | 5 | Pocatello | 1893 | 1B | 82,839 | 1,113 | 2,883 |
| Bear Lake County | 7 | Paris | 1875 | 2B | 5,986 | 971 | 2,515 |
| Benewah County | 9 | St. Maries | 1915 | 3B | 9,285 | 776 | 2,010 |
| Bingham County | 11 | Blackfoot | 1885 | 4B | 45,607 | 2,095 | 5,426 |
| Blaine County | 13 | Hailey | 1895 | 5B | 21,376 | 2,645 | 6,851 |
| Boise County | 15 | Idaho City | 1864 | 6B | 7,028 | 1,902 | 4,926 |
| Bonner County | 17 | Sandpoint | 1907 | 7B | 40,877 | 1,738 | 4,501 |
| Bonneville County | 19 | Idaho Falls | 1911 | 8B | 104,234 | 1,869 | 4,841 |
| Boundary County | 21 | Bonners Ferry | 1915 | 9B | 10,972 | 1,269 | 3,287 |
| Butte County | 23 | Arco | 1917 | 10B | 2,891 | 2,233 | 5,783 |
| Camas County | 25 | Fairfield | 1917 | 1C | 1,117 | 1,075 | 2,784 |
| Canyon County | 27 | Caldwell | 1892 | 2C | 188,923 | 590 | 1,528 |
| Caribou County | 29 | Soda Springs | 1919 | 3C | 6,963 | 1,766 | 4,574 |
| Cassia County | 31 | Burley | 1879 | 4C | 22,952 | 2,567 | 6,648 |
| Clark County | 33 | Dubois | 1919 | 5C | 982 | 1,765 | 4,571 |
| Clearwater County | 35 | Orofino | 1911 | 6C | 8,761 | 2,462 | 6,377 |
| Custer County | 37 | Challis | 1881 | 7C | 4,368 | 4,926 | 12,758 |
| Elmore County | 39 | Mountain Home | 1889 | E | 27,038 | 3,078 | 7,972 |
| Franklin County | 41 | Preston | 1913 | 1F | 12,786 | 666 | 1,725 |
| Fremont County | 43 | Saint Anthony | 1893 | 2F | 13,242 | 1,867 | 4,836 |
| Gem County | 45 | Emmett | 1915 | 1G | 16,719 | 563 | 1,458 |
| Gooding County | 47 | Gooding | 1913 | 2G | 15,464 | 731 | 1,893 |
| Idaho County | 49 | Grangeville | 1864 | I | 16,267 | 8,485 | 21,976 |
| Jefferson County | 51 | Rigby | 1913 | 1J | 26,139 | 1,095 | 2,836 |
| Jerome County | 53 | Jerome | 1919 | 2J | 22,374 | 600 | 1,554 |
| Kootenai County | 55 | Coeur d'Alene | 1864 | K | 138,494 | 1,245 | 3,225 |
| Latah County | 57 | Moscow | 1888 | 1L | 37,244 | 1,077 | 2,789 |
| Lemhi County | 59 | Salmon | 1869 | 2L | 7,936 | 4,564 | 11,821 |
| Lewis County | 61 | Nezperce | 1911 | 3L | 3,821 | 479 | 1,241 |
| Lincoln County | 63 | Shoshone | 1895 | 4L | 5,208 | 1,206 | 3,124 |
| Madison County | 65 | Rexburg | 1913 | 1M | 37,536 | 472 | 1,222 |
| Minidoka County | 67 | Rupert | 1913 | 2M | 20,069 | 760 | 1,968 |
| Nez Perce County | 69 | Lewiston | 1864 | N | 39,265 | 849 | 2,199 |
| Oneida County | 71 | Malad City | 1864 | 1O | 4,286 | 1,200 | 3,108 |
| Owyhee County | 73 | Murphy | 1863 | 2O | 11,526 | 7,678 | 19,886 |
| Payette County | 75 | Payette | 1917 | 1P | 22,623 | 408 | 1,057 |
| Power County | 77 | American Falls | 1913 | 2P | 7,817 | 1,406 | 3,642 |
| Shoshone County | 79 | Wallace | 1864 | S | 12,765 | 2,634 | 6,822 |
| Teton County | 81 | Driggs | 1915 | 1T | 10,170 | 450 | 1,165 |
| Twin Falls County | 83 | Twin Falls | 1907 | 2T | 77,230 | 1,925 | 4,986 |
| Valley County | 85 | Cascade | 1917 | V | 9,862 | 3,733 | 9,668 |
| Washington County | 87 | Weiser | 1879 | W | 10,198 | 1,456 | 3,771 |
