The Northern Ireland Assembly (Irish: Tionól Thuaisceart Éireann; Ulster Scots: Norlin Airlan Assemblie), often referred to by the metonym Stormont, is the devolved unicameral legislature of Northern Ireland. It has power to legislate in a wide range of areas that are not explicitly reserved to the Parliament of the United Kingdom, and to appoint the Northern Ireland Executive. It sits at Parliament Buildings at Stormont in Belfast.

The Assembly is a unicameral, democratically elected body comprising 90 members known as members of the Legislative Assembly (MLAs). Members are elected under the single transferable vote form of proportional representation (STV-PR). In turn, the Assembly selects most of the ministers of the Northern Ireland Executive using the principle of power-sharing under the D'Hondt method to ensure that Northern Ireland's largest voting blocs, British unionists and Irish nationalists, both participate in governing the region. The Assembly's standing orders allow for certain contentious motions to require a cross-community vote; in addition to requiring the support of an overall majority of members, such votes must also be supported by a majority within both blocs in order to pass.

The Assembly is one of two "mutually inter-dependent" institutions created under the 1998 Good Friday Agreement, the other being the North/South Ministerial Council with the Republic of Ireland. The Agreement aimed to end Northern Ireland's violent 30-year Troubles. The first Assembly election was held in June 1998.

From June 1921 until March 1972, the devolved legislature for Northern Ireland was the Parliament of Northern Ireland, established by the Government of Ireland Act 1920 and meeting from 1932 at Stormont, outside Belfast. Due to gerrymandering practices, the Parliament always had an Ulster Unionist Party (UUP) majority and always elected a UUP administration. For its first two elections it used proportional representation (Single transferable voting) but switched to First-past-the-post voting in 1929.

It was suspended by the UK Government on 30 March 1972 and formally abolished in 1973 under the Northern Ireland Constitution Act 1973.

Northern Ireland was subsequently administered by direct rule until 1999, with a brief exception in 1974. Attempts began to restore on a new basis that would see power shared between nationalists and unionists. To this end a new legislature, the Northern Ireland Assembly, was established in 1973 with a power-sharing Executive taking office in January 1974. However, this body was brought down by the Ulster Workers' Council strike in May 1974. Political discussions continued against the continued backdrop of the Troubles. In 1982, another Northern Ireland Assembly was established, initially as a body to scrutinise the actions of the Northern Ireland Civil Service and the Secretary of State, the UK Government minister with responsibility for Northern Ireland. It was not supported by Irish nationalists and was officially dissolved in 1986.

The Northern Ireland (Elections) Act 1998 formally established the Assembly in law under the name New Northern Ireland Assembly, in accordance with the Good Friday (or Belfast) Agreement. The first election of members of the New Northern Ireland Assembly was on 25 June 1998 and it first met on 1 July 1998. However, it only existed in "shadow" form until 2 December 1999 when full powers were devolved to the Assembly. Since then the Assembly has operated with several interruptions and has been suspended on six occasions:

Attempts to secure its operation on a permanent basis were initially frustrated by disagreements between the two main unionist parties (the Democratic Unionist Party (DUP) and the Ulster Unionist Party) and Sinn Féin. Unionist representatives refused to participate in the Good Friday Agreement's institutions alongside Sinn Féin until they were assured that the IRA had discontinued its activities, decommissioned its weapons, and disbanded.