| County | FIPS Code | County Seat | Established in | Population (2019) | Land Area (sq mi) | Land Area (km2) |
|---|---|---|---|---|---|---|
| Adams County | 1 | Ritzville | 1883 | 19,983 | 1,925 | 4,986 |
| Asotin County | 3 | Asotin | 1883 | 22,582 | 636 | 1,647 |
| Benton County | 5 | Prosser | 1905 | 204,390 | 1,700 | 4,403 |
| Chelan County | 7 | Wenatchee | 1899 | 77,200 | 2,920 | 7,563 |
| Clallam County | 9 | Port Angeles | 1854 | 77,331 | 1,738 | 4,501 |
| Clark County | 11 | Vancouver | 1845 | 488,241 | 629 | 1,629 |
| Columbia County | 13 | Dayton | 1875 | 3,985 | 869 | 2,251 |
| Cowlitz County | 15 | Kelso | 1854 | 110,593 | 1,139 | 2,950 |
| Douglas County | 17 | Waterville | 1883 | 43,429 | 1,819 | 4,711 |
| Ferry County | 19 | Republic | 1899 | 7,627 | 2,204 | 5,708 |
| Franklin County | 21 | Pasco | 1883 | 95,222 | 1,242 | 3,217 |
| Garfield County | 23 | Pomeroy | 1881 | 2,225 | 710 | 1,839 |
| Grant County | 25 | Ephrata | 1909 | 97,733 | 2,680 | 6,941 |
| Grays Harbor County | 27 | Montesano | 1854 | 75,061 | 1,902 | 4,926 |
| Island County | 29 | Coupeville | 1852 | 85,141 | 209 | 541 |
| Jefferson County | 31 | Port Townsend | 1852 | 32,221 | 1,804 | 4,672 |
| King County | 33 | Seattle | 1852 | 2,252,782 | 2,115 | 5,478 |
| Kitsap County | 35 | Port Orchard | 1857 | 271,473 | 395 | 1,023 |
| Kittitas County | 37 | Ellensburg | 1883 | 47,935 | 2,297 | 5,949 |
| Klickitat County | 39 | Goldendale | 1859 | 22,425 | 1,872 | 4,848 |
| Lewis County | 41 | Chehalis | 1845 | 80,707 | 2,403 | 6,224 |
| Lincoln County | 43 | Davenport | 1883 | 10,939 | 2,311 | 5,985 |
| Mason County | 45 | Shelton | 1854 | 66,768 | 959 | 2,484 |
| Okanogan County | 47 | Okanogan | 1888 | 42,243 | 5,268 | 13,644 |
| Pacific County | 49 | South Bend | 1851 | 22,471 | 933 | 2,416 |
| Pend Oreille County | 51 | Newport | 1911 | 13,724 | 1,400 | 3,626 |
| Pierce County | 53 | Tacoma | 1852 | 904,980 | 1,670 | 4,325 |
| San Juan County | 55 | Friday Harbor | 1873 | 17,582 | 174 | 451 |
| Skagit County | 57 | Mount Vernon | 1883 | 129,205 | 1,731 | 4,483 |
| Skamania County | 59 | Stevenson | 1854 | 12,083 | 1,656 | 4,289 |
| Snohomish County | 61 | Everett | 1861 | 822,083 | 2,087 | 5,405 |
| Spokane County | 63 | Spokane | 1879 | 522,798 | 1,764 | 4,569 |
| Stevens County | 65 | Colville | 1863 | 45,723 | 2,478 | 6,418 |
| Thurston County | 67 | Olympia | 1852 | 290,536 | 722 | 1,870 |
| Wahkiakum County | 69 | Cathlamet | 1854 | 4,488 | 264 | 684 |
| Walla Walla County | 71 | Walla Walla | 1854 | 60,760 | 1,270 | 3,289 |
| Whatcom County | 73 | Bellingham | 1854 | 229,247 | 2,107 | 5,457 |
| Whitman County | 75 | Colfax | 1871 | 50,104 | 2,159 | 5,592 |
| Yakima County | 77 | Yakima | 1865 | 250,873 | 4,296 | 11,127 |
