The Alliance Party of Northern Ireland (APNI), or simply Alliance, a liberal and centrist political party in Northern Ireland, is founded on April 21, 1970, with Phelim O’Neill as leader.

As of the 2022 Northern Ireland Assembly election, the Alliance Party is the third-largest party in the Northern Ireland Assembly, holding seventeen seats, and has made recent breakthroughs to place third in first preference votes in the 2019 European Parliament election and third highest-polling regionally at the 2019 United Kingdom general election. The party wins one of the three Northern Ireland seats in the European Parliament, and one seat, North Down, in the House of Commons, the lower house of the Parliament of the United Kingdom.

Founded in 1970 from the New Ulster Movement, the Alliance Party originally represents moderate and non-sectarian unionism. However, over time, particularly in the 1990s, it moves towards neutrality on the Union, and has come to represent wider liberal and non-sectarian concerns. It supports the Good Friday Agreement but maintains a desire for the reform of the political system toward a non-sectarian future. In the Northern Ireland Assembly, it is designated as neither unionist nor Irish nationalist, but “Other.”

The Alliance Party wins their first seat in the UK House of Commons in the 2010 United Kingdom general election, unseating the former Belfast East MP Peter Robinson, First Minister of Northern Ireland and leader of the Democratic Unionist Party (DUP). Naomi Long is the first MP from the Alliance Party since Stratton Mills, who joined the party from the Ulster Unionist Party (UUP) in 1973. However, the DUP regains the seat at the 2015 United Kingdom general election following an electoral pact with the UUP. In the 2019 United Kingdom general election, the Alliance Party regains its presence in the House of Commons when Stephen Farry wins the North Down seat vacated by the independent unionist, Sylvia Hermon. Earlier in the year, the party’s leader, Naomi Long, wins the party’s first seat in the European Parliament in the last European election before Brexit. Under Long’s leadership, the Alliance Party exceeds expectations in the 2022 Northern Ireland Assembly election and gains numerous seats in the Northern Ireland Assembly.

The Alliance Party is a member of Liberal International and the Alliance of Liberals and Democrats for Europe Party and is aligned with the Liberal Democrats in Great Britain.