The 1958 United States Senate election in Montana took place on November 4, 1958. Incumbent United States Senator Mike Mansfield, who was first elected to the Senate in 1952, ran for re-election. Mansfield won the Democratic primary comfortably, and moved on to the general election, where he was opposed by Lou W. Welch, a millworker and the Republican nominee. In contrast to the close campaign in 1952, Mansfield defeated Welch in a landslide and won his second term in the Senate easily.
- Mike Mansfield, incumbent United States Senator
- J. M. Nickey
- Thomas G. Stimatz, former State Representative
|Democratic||Mike Mansfield (inc.)||97,207||91.72|
|Democratic||J. M. Nickey||4,710||4.44|
|Democratic||Thomas G. Stimatz||4,061||3.83|
- Lou W. Welch, millworker
- Blanche Anderson
|Republican||Lou W. Welch||19,860||50.30|
|Democratic||Mike Mansfield (inc.)||174,910||76.22%||+25.47%|
|Republican||Lou W. Welch||54,573||23.78%||-24.77%|
- "Report of the Official Canvass of the Vote Cast at the Primary Election Held in the State of Montana, June 3, 1958" (PDF). Montana Secretary of State. Archived from the original (PDF) on December 28, 2013. Retrieved July 2, 2014.
- "Statistics of the Presidential and Congressional Election of November 4, 1958" (PDF). Clerk of the United States House of Representatives. Retrieved July 4, 2014.