The 1978 United States Senate election in Montana took place on November 7, 1978. Following the death of United States Senator Lee Metcalf on January 12, 1978, Montana Supreme Court Chief Justice Paul G. Hatfield was appointed to serve for the remainder of Metcalf's term. Hatfield opted to run for a full term, but was overwhelmingly defeated in the Democratic primary by U.S. Representative Max Baucus of the 1st congressional district. Baucus advanced to the general election, where he was opposed by the Republican nominee, author Larry R. Williams. Baucus ended up defeating Williams by a solid margin to win his first term in the Senate, and, following Hatfield's resignation on December 12, 1978, he began serving his first term in the Senate.
Despite the death of Metcalf, this was not a special election, as one for the year 1978 was already planned as a normal Senate election.