PO | County | Dates | Comments |
| JACKSON | Cassia | 1906-1916 | |
| JAMESTOWN | Latah | 1883-1890 | |
| JANSVILLE | Latah | 1892-1907 | |
| JENKINS | Shoshone | 1903-1905 | |
| JEROME | Jerome | 1907-Date | |
| JERUSALEM | Boise | 1885-1888 | |
| JESSIE | Cassia | 1882-1887 | |
| 1888-1905 | |||
| JOHN DAY'S CREEK | Idaho | 1866-1870 | |
| 1870-1885 | |||
| JORDAN CREEK | Custer | 1876-1879 | was ROBINSON BAR |
| renamed BONANZA CITY | |||
| JOSEPH | Clearwater | 1899-1899 | renamed AHSAHKA |
| JOSEPH | Idaho | 1905-1944 | |
| JULIAETTA | Latah | 1882-Date | |
| JUMBO | Idaho | 1912-1913 | |
| 1914-1920 | |||
| JUNCTION | Lemhi | 1874-1919 | |
| JUNIPER | Oneida | 1914-1943 | |
| JUNIPER | Owyhee | 1895-1896 |
Contributed 2021 Oct 05 by Mark Metkin
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