| Name | Created | Created from | County seat | |
| Adair | 1851 | Cass | Greenfield | |
| Adams | 1851 | Taylor | Corning | |
| Allamakee | 1847 | Clayton | Waukon | |
| Appanoose | 1843 | Davis | Centerville | |
| Audubon | 1851 | Cass, Black Hawk | Audubon | |
| Bancroft | 1851 | extinct | ||
| Benton | 1837 | Indian Land Purchase; Wisconsin Territory | Vinton | |
| Black Hawk | 1843 | Delaware | Waterloo | |
| Boone | 1846 | Polk | Boone | |
| Bremer | 1851 | Winnebago, Indian Reserve | Waverly | |
| Buchanan | 1837 | Delaware, Wisconsin Territory | Independence | |
| Buncombe | changed to Lyon,1862 | |||
| Buena Vista | 1851 | Sac, Clay | Storm Lake | |
| Butler | 1851 | Buchanan, Black Hawk | Allison | |
| Calhoun | 1855 | formerly called Fox | Rockwell City | |
| Carroll | 1851 | Guthrie | Carroll | |
| Cass | 1851 | Pottawattamie | Atlantic | |
| Cedar | 1837 | Wisconsin Territory | Tipton | |
| Cerro Gordo | 1851 | Floyd | Mason City | |
| Cherokee | 1851 | Crawford | Cherokee | |
| Chickasaw | 1851 | Fayette | New Hampton | |
| Clarke | 1846 | Lucas | Osceola | |
| Clay | 1851 | Indian Lands | Spencer | |
| Clayton | 1837 | Dubuque, Wisconsin Territory | Elkader | |
| Clinton | 1837 | Dubuque, Wisconsin Territory | Clinton | |
| Cook County | 1836 | from Demoin & Dubuque; extinct | ||
| Crawford | 1851 | Shelby | Denison | |
| Dallas | 1846 | Polk | Adel | |
| Davis | 1843 | Van Buren | Bloomfield | |
| Decatur | 1846 | Appanoose | Leon | |
| Delaware | 1837 | Dubuque, Wisconsin Territory | Manchester | |
| Des Moines (Originally Demoine) | 1834 | Michigan Territory; 1836 Wisconsin Territory | Burlington | |
| Dickinson | 1851 | Kossuth | Spirit Lake | |
| Dubuque | 1834 | Michigan Territory; 1836 Wisconsin Territory | Dubuque | |
| Emmet | 1851 | Kossuth, Dickinson | Estherville | |
| Fayette | 1837 | Clayton, Wisconsin Territory | West Union | |
| Floyd | 1851 | Chickasaw | Charles City | |
| Fox | 1851 | 1855, renamed Calhoun | ||
| Franklin | 1851 | Chickasaw | Hampton | |
| Fremont | 1847 | Pottawattamie | Sidney | |
| Greene | 1851 | Dallas | Jefferson | |
| Grundy | 1851 | Black Hawk | Grundy Center | |
| Guthrie | 1851 | Jackson | Guthrie Center | |
| Hamilton | 1856 | Webster | Webster City | |
| Hancock | 1851 | Wright | Garner | |
| Hardin | 1851 | Black Hawk | Eldora | |
| Harrison | 1851 | Pottawattamie | Logan | |
| Henry | 1836 | Wisconsin Territory | Mount Pleasant | |
| Howard | 1851 | Chickasaw, Floyd | Cresco | |
| Humboldt | 1851 | Webster (originally spelled Humbolt) | Dakota City | |
| Ida | 1851 | Cherokee | Ida Grove | |
| Iowa | 1843 | Washington | Marengo | |
| Jackson | 1837 | Wisconsin Territory | Maquoketa | |
| Jasper | 1846 | Mahaska | Newton | |
| Jefferson | 1839 | Indian Land Purchase | Fairfield | |
| Johnson | 1837 | Des Moines, Wisconsin Territory | Iowa City | |
| Jones | 1837 | Wisconsin Territory | Anamosa | |
| Keokuk | 1837 | Wisconsin Territory; extinct | ||
| Keokuk | 1843 | Washington | Sigourney | |
| Kishkekosh | changed to Monroe, 1846 | |||
| Kossuth | 1851 | Webster | Algona | |
| Lee | 1836 | Des Moines | Fort Madison | |
| Linn | 1837 | Wisconsin Territory | Cedar Rapids | |
| Louisa | 1836 | Des Moines | Wapello | |
| Lucas | 1846 | Monroe | Chariton | |
| Lyon | 1851 | Woodbury | Rock Rapids | |
| Madison | 1844 | extinct | ||
| Madison | 1846 | Polk | Winterset | |
| Mahaska | 1843 | Fox, Sac Indian Purchase | Oskaloosa | |
| Marion | 1845 | Washington | Knoxville | |
| Marshall | 1846 | Jasper | Marshalltown | |
| Mills | 1851 | Pottawattamie | Glenwood | |
| Mitchell | 1851 | Chickasaw | Osage | |
| Monona | 1851 | Harrison | Onawa | |
| Monroe | 1843 | Monroe (formerly Kishkekosh) | Albia | |
| Montgomery | 1851 | Polk | Red Oak | |
| Muscatine | 1836 | Des Moines | Muscatine | |
| O'Brien | 1851 | Cherokee | Primghar | |
| Osceola | 1851 | Woodbury | Sibley | |
| Page | 1847 | Pottawattamie | Clarinda | |
| Palo Alto | 1858 | Kossuth | Emmetsburg | |
| Plymouth | 1851 | Woodbury | Le Mars | |
| Pocahontas | 1851 | Humboldt, Greene | Pocahontas | |
| Polk | 1846 | Indian Lands | Des Moines | |
| Pottawattamie | 1848 | Indian Lands | Council Bluffs | |
| Poweshiek | 1843 | Mesquakie Indian Lands | Montezuma | |
| Ringgold | 1847 | Taylor | Mount Ayr | |
| Risley | 1851 | changed to Webster 1853 | ||
| Sac | 1851 | Greene | Sac City | |
| Scott | 1837 | Wisconsin Territory | Davenport | |
| Shelby | 1851 | Cass | Harlan | |
| Sioux | 1851 | Plymouth | Orange City | |
| Slaughter | 1837 | Wisconsin Territory; renamed Washington, 1838 | ||
| Story | 1846 | Jasper, Polk, Boone | Nevada | |
| Tama | 1843 | Boone, Benton | Toledo | |
| Taylor | 1847 | Page | Bedford | |
| Union | 1851 | Clarke | Creston | |
| Van Buren | 1836 | Des Moines | Keosauqua | |
| Wahkaw | 1851 | changed to Woodbury, 1853 | ||
| Wapello | 1843 | Indian Lands | Ottumwa | |
| Warren | 1846 | Polk | Indianola | |
| Washington | 1837 | Wisconsin Territory; formerly Slaughter | Washington | |
| Wayne | 1846 | Appanoose | Corydon | |
| Webster | 1851 | Yell, Risley | Fort Dodge | |
| Winnebago | 1851 | Kossuth | Forest City | |
| Winneshiek | 1847 | Indian Lands | Decorah | |
| Woodbury | 1851 | Indian Lands (formerly Wahkaw) | Sioux City | |
| Worth | 1851 | Mitchell | Northwood | |
| Wright | 1851 | Webster | Clarion | |
| Yell | 1851 | extinct |