| Election Name: | Kensington and Chelsea by-election |
| Type: | presidential |
| Country: | United Kingdom |
| Seats For Election: | Kensington and Chelsea parliamentary seat |
| Ongoing: | no |
| Previous Election: | 1997 United Kingdom general election |
| Previous Year: | 1997 |
| Next Election: | 2001 United Kingdom general election |
| Next Year: | 2001 |
| Election Date: | 25 November 1999 |
| Turnout: | 29.7% |
| Candidate1: | Michael Portillo |
| Party1: | Conservative Party (UK) |
| Popular Vote1: | 11,004 |
| Percentage1: | 56.4% |
| Candidate2: | Robert Atkinson |
| Party2: | Labour Party (UK) |
| Popular Vote2: | 4,298 |
| Percentage2: | 22.0% |
| Candidate3: | Robert Woodthorpe Browne |
| Party3: | Liberal Democrats (UK) |
| Popular Vote3: | 1,831 |
| Percentage3: | 9.4% |
| MP | |
| Posttitle: | Subsequent MP |
| Before Election: | Alan Clark |
| Before Party: | Conservative Party (UK) |
| After Election: | Michael Portillo |
| After Party: | Conservative Party (UK) |
The 1999 Kensington and Chelsea by-election was held on 25 November 1999 after the member of Parliament for the constituency, Alan Clark (Conservative), died of a brain tumour on 5 September 1999.
This was the first safe Conservative seat to have a by-election in the 1997–2001 UK Parliament. There was immediate speculation that Michael Portillo, the most high-profile casualty of the 1997 general election, would use it to return to frontline politics. Portillo immediately confirmed his interest in the seat, but was then confronted with the publication of an interview he had given previously that summer in which he had confirmed that while at Peterhouse, Cambridge he had had homosexual affairs.
Portillo was selected as Conservative candidate but faced demonstrations organised by gay rights group OutRage! and its principal campaigner Peter Tatchell who protested against his vote for an unequal age of consent for gay and straight sex, and support for the ban on homosexuality in the UK armed forces while Secretary of State for Defence. Tatchell continued to try to confront Portillo throughout the election, not assuaged by Portillo saying that he had changed his mind on the age of consent.
The Labour Party selected Robert Atkinson, who had fought the 1997 election and was a local councillor. The Liberal Democrats also renominated their general election candidate, Robert Woodthorpe Browne. Because of the prominence of the by-election in central London and the big political name, there were a wide variety of fringe and minor party candidates. Polling day was 25 November. Portillo was returned safely to Parliament.