Idaho County is a county in the U.S. state of Idaho, and the largest by area in the state. As of the 2020 census, the population was 16,541. The county seat is Grangeville. Previous county seats of the area were Florence (1864–68), Washington (1868–75), and Mount Idaho (1875–1902).

Idaho is a landlocked state in the Mountain West subregion of the United States. It shares a small portion of the Canada–United States border to the north, with the province of British Columbia. It borders Montana and Wyoming to the east, Nevada and Utah to the south, and Washington and Oregon to the west. The state's capital and largest city is Boise. With an area of 83,569 square miles (216,440 km2), Idaho is the 14th largest state by land area. With a population of approximately 1.8 million, it ranks as the 13th least populous and the 7th least densely populated of the 50 U.S. states.