| Election Name: | 1990 Knowsley South by-election |
| Type: | parliamentary |
| Country: | United Kingdom |
| Seats For Election: | Constituency of Knowsley South |
| Ongoing: | no |
| Previous Election: | 1987 United Kingdom general election |
| Previous Year: | 1987 |
| Election Date: | 27 September 1990 |
| Candidate1: | Edward O'Hara |
| Party1: | Labour Party (UK) |
| Popular Vote1: | 14,581 |
| Percentage1: | 68.8% |
| Swing1: | 4.3% |
| Candidate2: | Leslie Byrom |
| Party2: | Conservative Party (UK) |
| Popular Vote2: | 3,241 |
| Percentage2: | 15.2% |
| Swing2: | 6.4% |
| Candidate3: | Catherin Hancox |
| Image3: | LD |
| Party3: | Liberal Democrats (UK) |
| Popular Vote3: | 1,809 |
| Percentage3: | 8.5% |
| Swing3: | 5.4% |
| MP | |
| Posttitle: | Subsequent MP |
| Before Election: | Sean Hughes |
| Before Party: | Labour Party (UK) |
| After Election: | Edward O'Hara |
| After Party: | Labour Party (UK) |
| Turnout: | 33.4% (40.7%) |
| Next Election: | 1992 United Kingdom general election |
| Next Year: | 1992 |
The 1990 Knowsley South by-election was a by-election held on 27 September 1990 for the British House of Commons constituency of Knowsley South in Merseyside.
The by-election was caused by the death of the constituency's Labour Party Member of Parliament (MP) Sean Hughes on 25 June.
The result was a Labour hold, with Edward O'Hara winning a majority of over 11,000 votes.