| Election Name: | 1940 United States Senate election in Wyoming |
| Country: | Wyoming |
| Type: | presidential |
| Ongoing: | no |
| Previous Election: | 1934 United States Senate elections in Wyoming |
| Previous Year: | 1934 |
| Next Election: | 1946 United States Senate election in Wyoming |
| Next Year: | 1946 |
| Election Date: | November 5, 1940 |
| Image1: | Sen. Joseph O'Mahoney, LCCN2016876768 (cropped).jpg |
| Nominee1: | Joseph C. O'Mahoney |
| Party1: | Democratic Party (United States) |
| Popular Vote1: | 65,022 |
| Percentage1: | 58.74% |
| Nominee2: | Milward Simpson |
| Party2: | Republican Party (United States) |
| Popular Vote2: | 45,682 |
| Percentage2: | 41.26% |
| Map Size: | 250px |
| U.S. Senator | |
| Before Election: | Joseph C. O'Mahoney |
| Before Party: | Democratic Party (United States) |
| After Election: | Joseph C. O'Mahoney |
| After Party: | Democratic Party (United States) |
The 1940 United States Senate election in Wyoming took place on November 5, 1940. Democratic Senator Joseph C. O'Mahoney ran for re-election to a second full term. He faced Republican Milward Simpson, a member of the University of Wyoming Board of Trustees and a former State Representative, in the general election. Though the presidential election in Wyoming was relatively close, O'Mahoney outperformed President Franklin D. Roosevelt's narrow win, and defeated Simpson in a landslide to win his second term.