| Election Name: | 1918 United States Senate election in South Dakota |
| Country: | South Dakota |
| Flag Year: | 1909 |
| Type: | presidential |
| Ongoing: | no |
| Previous Election: | 1913 United States Senate election in South Dakota |
| Previous Year: | 1913 |
| Next Election: | 1924 United States Senate election in South Dakota |
| Next Year: | 1924 |
| Election Date: | November 5, 1918 |
| Image1: | File:STERLING, THOMAS. SENATOR LCCN2016858418 (cropped).jpg |
| Nominee1: | Thomas Sterling |
| Party1: | Republican Party (United States) |
| Popular Vote1: | 51,198 |
| Percentage1: | 55.07% |
| Nominee2: | Orville V. Rinehart |
| Party2: | Democratic Party (United States) |
| Popular Vote2: | 36,210 |
| Percentage2: | 38.95% |
| Nominee3: | W. T. Rafferty |
| Party3: | Independent (politician) |
| Popular Vote3: | 5,560 |
| Percentage3: | 5.98% |
| Map Size: | 240px |
| U.S. Senator | |
| Before Election: | Thomas Sterling |
| Before Party: | Republican Party (United States) |
| After Election: | Thomas Sterling |
| After Party: | Republican Party (United States) |
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.[3]