| Election Name: | 1940 United States Senate election in Washington |
| Country: | Washington |
| Flag Image: | Flag of Washington (1923–1967).svg |
| Type: | Presidential |
| Ongoing: | no |
| Previous Election: | 1934 United States Senate election in Washington |
| Previous Year: | 1934 |
| Next Election: | 1946 United States Senate election in Washington |
| Next Year: | 1946 |
| Election Date: | November 5, 1940 |
| Image1: | File:Governor Monrad Charles Wallgren (cropped).jpg |
| Nominee1: | Monrad Wallgren |
| Party1: | Democratic Party (United States) |
| Popular Vote1: | 404,718 |
| Percentage1: | 54.16% |
| Nominee2: | Stephen F. Chadwick |
| Party2: | Republican Party (United States) |
| Popular Vote2: | 342,589 |
| Percentage2: | 45.84% |
| Map Size: | 270px |
| U.S. Senator | |
| Before Party: | Democratic Party (US) |
| After Party: | Democratic Party (US) |
The 1940 United States Senate election in Washington was held on November 5, 1940. Incumbent Democrat Lewis Schwellenbach did not run for a second term in office. He was succeeded by Democratic U.S. Representative Monrad C. Wallgren, who defeated Republican Stephen Foster Chadwick for the open seat.