| Election Name: | 1986 South Down by-election |
| Type: | parliamentary |
| Country: | United Kingdom |
| Seats For Election: | South Down constituency |
| Ongoing: | no |
| Previous Election: | 1983 United Kingdom general election |
| Previous Year: | 1983 |
| Next Election: | 1987 United Kingdom general election |
| Next Year: | 1987 |
| Candidate1: | Enoch Powell |
| Party1: | Ulster Unionist Party |
| Popular Vote1: | 24,963 |
| Percentage1: | 48.4% |
| Swing1: | 8.1% |
| Candidate2: | Eddie McGrady |
| Party2: | Social Democratic and Labour Party |
| Popular Vote2: | 23,121 |
| Percentage2: | 44.8 |
| Swing2: | 5.5% |
| Map Size: | 200px |
| MP | |
| Before Party: | Ulster Unionist Party |
| After Election: | Enoch Powell |
| After Party: | Ulster Unionist Party |
The South Down by-election of 1986 was part of a co-ordinated series of by-elections aimed to show opposition to the Anglo Irish agreement. It resulted in the narrow victory of the incumbent Ulster Unionist Party, Enoch Powell, who increased his majority by 1,294 votes despite rising support for Irish Nationalist candidates. His opponent Eddie McGrady would later take the seat in the 1987 general election.