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, one of two United States Representatives from Montana's at-large congressional district and the nominee of the National Party. Walsh narrowly won his second term in the Senate.
- Thomas J. Walsh, incumbent United States Senator
|Democratic||Thomas J. Walsh (inc.)||28,553||100.00%|
- Oscar M. Lanstrum, former State Representative
- Jeannette Rankin, one of two Congressional representatives from Montana's at-large congressional district
- Harry H. Parsons, attorney
- Edmund Nichols
|Republican||Oscar M. Lanstrum||18,805||41.59%|
|Republican||Harry H. Parsons||5,878||13.00%|
|Democratic||Thomas J. Walsh (inc.)||46,160||41.07%||-0.10%|
|Republican||Oscar M. Lanstrum||40,229||35.79%||+9.06%|
- "Report of the Official Canvass of the Vote Cast at the Primary Election Held in the State of Montana, August 27, 1918". Montana Secretary of State. Retrieved July 2, 2014.
- "Official Abstract of Votes Cast at the General Election Held in Montana, November 5, 1918" (PDF). Montana Secretary of State. Retrieved July 2, 2014.