| Election Name: | 1940 United States Senate special election in Illinois |
| Country: | Illinois |
| Flag Year: | 1915 |
| Type: | presidential |
| Ongoing: | no |
| Previous Election: | 1936 United States Senate election in Illinois |
| Previous Year: | 1936 |
| Next Election: | 1942 United States Senate election in Illinois |
| Next Year: | 1942 |
| Election Date: | November 5, 1940 |
| Image1: | Charles W. Brooks (Illinois Blue Book Portrait 1941-1942) (cropped).png |
| Nominee1: | Charles W. Brooks |
| Party1: | Republican Party (United States) |
| Popular Vote1: | 2,045,924 |
| Percentage1: | 50.07% |
| Nominee2: | James M. Slattery |
| Party2: | Democratic Party (United States) |
| Popular Vote2: | 2,025,097 |
| Percentage2: | 49.56% |
| Map Size: | 300px |
| U.S. senator | |
| Before Election: | James M. Slattery |
| Before Party: | Democratic Party (United States) |
| After Election: | Charles W. Brooks |
| After Party: | Republican Party (United States) |
The 1940 United States Senate special election in Illinois took place on November 5, 1940.[1] The election was triggered by the vacancy left by the death in office of Democrat J. Hamilton Lewis. After Lewis' death, James M. Slattery was appointed to fill the seat in the interim period until the individual elected in the special election would be sworn-in. Slattery was the Democratic Party's nominee in the special election. He was defeated by Republican nominee Charles W. Brooks. Brooks' father-in-law, Senator John Thomas of Idaho also won a special election that same day and the two would serve in the Senate together until Thomas' death in 1945.
The primaries and general election coincided with those for other federal elections (president and House) and those for state elections.[1]
Primaries were held April 9, 1940.[1]