Idaho County is a county located in the U.S. state of Idaho. As of the 2010 census, the population was 16,267. The county seat is Grangeville. Previous county seats of the area were Florence (1864–75) and Mount Idaho (1875–1902).

Idaho County was originally founded 155 years ago as a region of Washington Territory in 1861, named for a steamer called Idaho that was launched on the Columbia River in 1860. It was reorganized by the Idaho Territorial Legislature on February 4, 1864. In this context, the Idaho Territory and the State of Idaho are both preceded by the name of the county.