The 1986 Northern Ireland by-elections were fifteen 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, signed the month before.

Quick Facts 15 of 17 Northern Irish seats in the United Kingdom Parliament, First party ...

1986 Northern Ireland by-elections

← 1983 23 January 1986 1987 →
15 of 17 Northern Irish seats in the United Kingdom Parliament
First party Second party   Party Ulster Unionist DUP Seats won 10 3 Seat change 1 Popular vote 302,198 85,239 Percentage 51.7% 23.0% Swing 13.1% 5.5%   Third party Fourth party   Party SDLP UPUP Seats won 1 1 Seat change 1 Popular vote 70,917 30,793 Percentage 12.1% 5.3% Swing 3.1% 1.9%
Left map shows the % of the vote received by resigning unionist anti-agreement candidates. Right map shows the % of the vote received by pro-agreement candidates. Bottom map shows the winning party by constituency - in all constituencies but Armagh and Newry, resigning unionist candidates were re-elected.

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Each of the Unionist parties agreed not to contest seats previously held by the others, and each outgoing MP stood for re-election. 14 of the 15 were re-elected, often with large margins. The sole exception was in the Newry and Armagh by-election, where Seamus Mallon of the Social Democratic and Labour Party unseated Jim Nicholson (UUP).

Of the two Northern Ireland seats not held by unionists, Foyle was held by the nationalist Social Democratic and Labour Party (SDLP), while Belfast West was held by the republican party Sinn Féin. These MPs did not resign and their seats were not contested.

The SDLP and Sinn Féin regarded the resignations as a publicity stunt, and were reluctant to take part in the resulting by-elections. In the event, they contested only the seats which they believed to have an anti-unionist majority.

The Alliance Party of Northern Ireland instructed its branches to nominate a candidate for each by-election, but many were reluctant, and ultimately, the party stood in only five seats. The small Workers' Party stood in nine seats.

It appeared that the incumbent would be the only candidate in four constituencies. This would have produced the first uncontested elections for the House of Commons since the Armagh by-election of 1954. In order to ensure that there was a contest and that the Unionists would be able to demonstrate their point, Wesley Robert Williamson changed his name by deed poll to "Peter Barry", Peter Barry being Minister for Foreign Affairs of the Republic of Ireland. "Peter Barry" stood in these four constituencies under the label "For the Anglo-Irish Agreement", allowing a contest, but did not campaign.

The unusual circumstances led this to be the greatest number of UK parliamentary by-elections held on a single day.

• Lists of United Kingdom by-elections
• United Kingdom by-election records
• Five Constituencies Referendum
• July 2018 Australian federal by-elections