| Election Name: | 1963 Leeds South by-election |
| Type: | parliamentary |
| Country: | United Kingdom |
| Seats For Election: | Constituency of Leeds South |
| Ongoing: | no |
| Previous Election: | 1959 United Kingdom general election |
| Previous Year: | 1959 |
| Election Date: | 20 June 1963 |
| Candidate1: | Merlyn Rees |
| Party1: | Labour Party (UK) |
| Popular Vote1: | 18,785 |
| Percentage1: | 62.93% |
| Swing1: | 4.37% |
| Candidate2: | John Udal |
| Party2: | Conservative Party (UK) |
| Popular Vote2: | 5,996 |
| Percentage2: | 20.09% |
| Swing2: | 10.95% |
| Candidate3: | Brian Walsh |
| Image3: | Lib |
| Party3: | Liberal Party (UK) |
| Popular Vote3: | 4,399 |
| Percentage3: | 14.74% |
| Swing3: | 4.34% |
| MP | |
| Posttitle: | Subsequent MP |
| Before Election: | Hugh Gaitskell |
| Before Party: | Labour Party (UK) |
| After Election: | Merlyn Rees |
| After Party: | Labour Party (UK) |
| Map Size: | 200px |
| Next Election: | 1964 United Kingdom general election |
| Next Year: | 1964 |
The 1963 Leeds South by-election was held on Thursday 20 June 1963. It was held due to the death of the incumbent MP and Leader of the Labour Party, Hugh Gaitskell. The by-election was won by the Labour candidate, Merlyn Rees who would later become a Cabinet minister.