The 1918 United States Senate election in Montana took place on November 5, 1918. Incumbent United States Senator Thomas J. Walsh, who was first elected to the Senate in 1912, ran for re-election. He won the Democratic primary uncontested, and was opposed in the general election by Oscar M. Lanstrum, a former State Representative and the Republican nominee, and Jeannette Rankin, the United States Congresswoman from Montana's at-large congressional district and the National Party nominee. Ultimately, Walsh narrowly won his second term in the Senate.