| Country: | New South Wales |
| Type: | parliamentary |
| Ongoing: | no |
| Election Date: | 3 September 1921 |
| Vote Type: | Popular |
| Image1: | William Lambert.jpg |
| Candidate1: | William Lambert |
| Party1: | Australian Labor Party |
| Popular Vote1: | 7,857 |
| Percentage1: | 57.3% |
| Swing1: | 14.9pp |
| Candidate2: | Alfred Henry |
| Party2: | Nationalist Party (Australia) |
| Popular Vote2: | 5,237 |
| Percentage2: | 38.2% |
| Swing2: | 10.4pp |
| MP | |
| Before Election: | T. J. Ryan |
| Before Party: | Australian Labor Party |
| After Election: | William Lambert |
| After Party: | Australian Labor Party |
| Election Name: | 1921 West Sydney by-election |
| 1Data2: | 40.4% |
| 2Blank: | TPP swing |
| 2Data1: | 12.6pp |
| 2Data2: | 12.6pp |
| 1Data1: | 59.6% |
| 1Blank: | TPP |
A by-election was held for the Australian House of Representatives seat of West Sydney on 3 September 1921. This was triggered by the death of Labor MP T. J. Ryan.
The by-election was won by Labor candidate William Lambert.