The 1918 United States Senate election in South Dakota took place on November 5, 1918. Incumbent Republican Senator Thomas Sterling sought re-election in his first popular election. He defeated former Governor Frank M. Byrne in the Republican primary and then faced former State Representative Orville Rinehart, the 1916 Democratic nominee for Governor, in the general election. Sterling defeated Rinehart, along with independent candidate W. T. Rafferty, by a wide margin to win re-election.
After winning the primary, Knapp withdrew as a candidate, citing his disagreement with the Socialist Party's anti-war policies.