| Election Name: | 1986 Newry and Armagh by-election |
| Type: | parliamentary |
| Country: | United Kingdom |
| Seats For Election: | Constituency of Newry and Armagh |
| Ongoing: | no |
| Previous Election: | 1983 United Kingdom general election |
| Previous Year: | 1983 |
| Election Date: | 23 January 1986 |
| Candidate1: | Seamus Mallon |
| Party1: | Social Democratic and Labour Party |
| Popular Vote1: | 22,694 |
| Percentage1: | 45.5% |
| Swing1: | 8.7% |
| Candidate2: | Jim Nicholson |
| Party2: | Ulster Unionist Party |
| Popular Vote2: | 20,111 |
| Percentage2: | 40.3% |
| Swing2: | 0.3% |
| Candidate3: | Jim McAllister |
| Image3: | SF |
| Party3: | Sinn Féin |
| Popular Vote3: | 6,609 |
| Percentage3: | 13.2% |
| Swing3: | 7.7% |
| MP | |
| Posttitle: | Subsequent MP |
| Before Election: | Jim Nicholson |
| Before Party: | Ulster Unionist Party |
| After Election: | Seamus Mallon |
| After Party: | Social Democratic and Labour Party |
| Turnout: | 76.9% (0.9%) |
| Next Election: | 1987 United Kingdom general election |
| Next Year: | 1987 |
The 1986 by-election in Newry and Armagh was caused by the resignation of incumbent Member of Parliament Jim Nicholson.
Nicholson, along with all sitting Unionist MPs, resigned their Westminster seats in December 1985, to highlight their opposition to the Anglo-Irish Agreement and to use the resultant by-elections to campaign on the issue.
The poll was held on 23 January 1986, and was unusual for a by-election in the turnout being higher than for the preceding general election, however Nicholson was not re-elected, (despite gaining an increase in his own vote) losing his seat to the nationalist SDLP candidate Seamus Mallon. Nicholson also failed to recapture the seat at the following general election.