| Election Name: | 1986 Belfast South by-election |
| Type: | parliamentary |
| Country: | United Kingdom |
| Seats For Election: | Constituency of Belfast South |
| Ongoing: | no |
| Previous Election: | 1983 United Kingdom general election |
| Previous Year: | 1983 |
| Election Date: | 23 Jan 1986 |
| Candidate1: | Martin Smyth |
| Party1: | Ulster Unionist Party |
| Popular Vote1: | 21,771 |
| Percentage1: | 71.3% |
| Swing1: | 21.3% |
| Candidate2: | David Cook |
| Party2: | Alliance Party of Northern Ireland |
| Popular Vote2: | 7,635 |
| Percentage2: | 25.0% |
| Swing2: | 1.1% |
| MP | |
| Posttitle: | Subsequent MP |
| Before Election: | Martin Smyth |
| Before Party: | Ulster Unionist Party |
| After Election: | Martin Smyth |
| After Party: | Ulster Unionist Party |
| Turnout: | 56.9% (12.7%) |
| Next Election: | 1987 United Kingdom general election |
| Next Year: | 1987 |
The 1986 Belfast South by-election was one of the fifteen 1986 Northern Ireland by-elections held on 23 January 1986, to fill vacancies in the Parliament of the United Kingdom caused by the resignation in December 1985 of all sitting Unionist Members of Parliament (MPs). The MPs, from the Ulster Unionist Party, Democratic Unionist Party and Ulster Popular Unionist Party, did this to highlight their opposition to the Anglo-Irish Agreement. Each of their parties agreed not to contest seats previously held by the others, and each outgoing MP stood for re-election.