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.
