Lewis and Clark County is a county located in the U.S. state of Montana. As of the 2020 census, the population was 70,973. Its county seat is Helena, the state capital. The numerical designation for Lewis and Clark County is 5. The county was established on June 2, 1865, as one of the nine original counties of the Territory of Montana named Edgerton County in honor of Sidney Edgerton, first Governor of the Territory of Montana, and was renamed Lewis and Clark County on March 1, 1868, in honor of explorers Meriwether Lewis and William Clark.

Helena is the capital city of the U.S. state of Montana and the seat of Lewis and Clark County.