Lemhi County is a county located in the U.S. state of Idaho. As of the 2020 census, the population was 7,974. The largest city and county seat is Salmon. The county was established in 1869 and named after Fort Lemhi, a remote Mormon missionary settlement from 1855 to 1858 in Bannock and Shoshone territory.

Salmon is a city in Lemhi County, Idaho. The population was 3,112 at the 2010 census. The city is the county seat of Lemhi County.