| Country: | Western Australia |
| Type: | parliamentary |
| Ongoing: | no |
| Election Date: | 21 December 1940 |
| Vote Type: | Popular |
| Image1: | Thomas William Marwick.JPG |
| Candidate1: | Thomas Marwick |
| Party1: | Country |
| Popular Vote1: | 12,354 |
| Percentage1: | 27.6% |
| Swing1: | 1.6pp |
| Candidate2: | Jim Dinan |
| Party2: | Australian Labor Party |
| Popular Vote2: | 16,729 |
| Percentage2: | 37.4% |
| Swing2: | 9.2pp |
| MP | |
| Before Election: | Henry Gregory |
| Before Party: | Country |
| After Election: | Thomas Marwick |
| After Party: | Country |
| Election Name: | 1940 Swan by-election |
| 1Data2: | 47.3% |
| 2Blank: | TPP swing |
| 2Data1: | 5.2pp |
| 2Data2: | 5.2pp |
| 1Data1: | 52.3% |
| 1Blank: | TPP |
| Image4: | Percy Ferguson.jpg |
| Candidate4: | Percy Ferguson |
| Party4: | Country |
| Popular Vote4: | 10,628 |
| Percentage4: | 23.8% |
| Swing4: | 23.8pp |
| Candidate5: | Claude Barker |
| Party5: | Independent |
| Popular Vote5: | 4,980 |
| Percentage5: | 11.1% |
| Swing5: | 11.1pp |
| Colour5: | 888888 |
A by-election was held for the Australian House of Representatives seat of Swan on 21 December 1940. This was triggered by the death of Country Party MP Henry Gregory.
The by-election was won by Country Party candidate Thomas Marwick, who had been a member of the Senate from 1936 to 1937.