The Ulster Unionist Party (UUP, sometimes referred to as the Official Unionist Party or OUP or, in a historic sense, simply the Unionist Party ) is a moderate unionist political party in Northern Ireland, which formed its government between 1921 and 1972.

The Ulster Unionist Party traces its formal existence back to the foundation of the Ulster Unionist Council in 1905. Prior to that date, however, there had been a less formally-organised Irish Unionist Party since the late 19th century, sometimes but not always dominated by Unionists from Ulster. Modern organised unionism properly emerged in response to Gladstone's introduction of the First Home Rule Bill in 1886 in response to demands by the Irish Parliamentary Party. The Irish Unionist Party was an alliance of Conservatives and Liberal Unionists who had split from the Liberal Party on the home rule issue.

The party had a strong association with the politico-religious Orange Order, a Protestant institution which is the counterpart of the smaller Roman Catholic Ancient Order of Hibernians. Though most unionist support was based in the geographic area that became Northern Ireland, there were at one time Unionist enclaves throughout southern Ireland. The Unionists in Cork and Dublin were particularly influential. The initial leadership of the Unionist Party all came from outside the six counties, with people like Colonel Saunderson, Viscount (later the Earl of) Midleton and the Dublin-born Sir Edward Carson. However, with the partition of Ireland under the Government of Ireland Act 1920, Irish unionism in effect split. Many southern unionists became reconciled with the new Irish Free State, many sitting in its senate or joining its political parties. Unionism's northern wing evolved into a separate Ulster Unionist Party.

Modern southern unionism has evolved into the Reform Movement in Ireland and the Irish Unionist Alliance. Both organizations are separate from the Ulster Unionist Party.

Until almost the very end of its period of power in Northern Ireland the Unionist Party was led by a combination of landed gentry (Sir Basil Brooke [later Lord Brookeborough], Terence O'Neill and James Chichester-Clark) and gentrified industrial magnates (Sir James Craig later Lord Craigavon, and John Miller Andrews). Only its last Prime Minister, Brian Faulkner was from a middle-class background.

In 1922, Sir Edward Carson warned the new unionist leadership of Northern Ireland against practising any discrimination towards the Roman Catholic minority in the region. It was advice that went unheeded. As former leader and Nobel Peace Prize co-winner (with the SDLP's then leader, John Hume) David Trimble observed, Northern Ireland under the UUP governments was a "cold house for Catholics." In the 1960s, inspired by the civil rights movement of Martin Luther King and by attempts at reform under then Unionist Party leader Terence O'Neill (later Lord O'Neill of the Maine) nationalists in the Northern Civil Rights Movement campaigned for reform. However, violent opposition from extreme loyalists and right-wing campaigners like Ian Paisley, coupled with the heavy-handled behaviour of the police, led to a resurgence in violence by the Provisional Irish Republican Army, a breakaway from the Marxist Official Irish Republican Army. Faced with what seemed like a threat of civil war, the British government ended the Unionist Party's hold on power in Northern Ireland, when it prorogued the Stormont Parliament in March 1972.

Some liberal Unionists, who had advocated the policies of Terence O'Neill left and formed the Alliance Party in 1970, while the emergence of Ian Paisley's Democratic Unionist Party (DUP) drew off some working-class and right-wing support. A more militant wing of the Unionist Party turned to the Vanguard movement to steer unionism back to its "traditional" course. When this failed, they broke away and formed the separate Vanguard Unionist Progressive Party in 1973.

Throughout this period the party was affiliated to the National Union of the Conservative Party and Ulster Unionist MPs at the Westminster Parliament were a part of the conservative block. To all intents and purposes the party functioned as the Northern Ireland branch of the Conservatives. (The names were different, but in the same period the Scottish branch of the party used the term "Unionist" instead of Conservative as well.) In 1974, in protest over the Sunningdale Agreement the Westminster Ulster Unionist MPs ceased to take the Conservative Party whip. The party remained affiliated to the National Union but withdrew in 1985 in protest over the Anglo-Irish Agreement. Subsequently the Conservative Party established a separate branch in Northern Ireland which has had little electoral success. There is frequent speculation that the Ulster Unionists may reunite with the Conservative Party, as the party has continued to support the Conservatives on essential votes. For example, Ulster Unionist support was necessary to sustain the Conservative Government of Prime Minister John Major during the ratification of the Maastricht Treaty in 1993 when many of his own MPs (the Maastricht Rebels) revolted. Many commentators felt this gave the UUP much influence on Major's Northern Ireland policy at the time.

The UUP maintained a formal connection with the Orange Order from its foundation until 2005, and with the Apprentice Boys of Derry until 1975. Only three of the party's Westminster MPs (Enoch Powell, Ken Maginnis and Sylvia Hermon) have not been members of the Orange Order. This was a factor in discouraging Catholic membership of the party. While the party was considering structural reforms, including the connection with the Order, it was the Order itself that severed the connection after many of its members transferred their allegiance to the Democratic Unionist Party.

The UUP's youth wing is the Ulster Young Unionist Council. An affiliated body of this name has been in existence since 1949 and has cultivated many hues of ideology and personalities who have either staid with the party: such as the present leader of the UUP or others who have left to start up other unionist parties. Having disbanded twice, 1974 and 2004, the Council was re-constituted by young activists in March 2004. This resulted in the young unionists (YU) becoming a representative body of the UUP and subject to its revamp of their Constitution.

The Sunningdale Agreement, which led to the formation of a power-sharing Executive under the then Ulster Unionist leader, Brian Faulkner provoked ruptions within the party. In the 1973 elections to the Executive the party was deeply divided, a division that did not formally end until January 1974 when the anti-Sunningdale faction triumphed. Faulkner was overthrown and set up the Unionist Party of Northern Ireland (UPNI). The Ulster Unionists were now led by Harry West from 1974 until 1979. In the February 1974 general election, the party participated in the United Ulster Unionist Coalition (UUUC) with Vanguard and the Democratic Unionists. The result was that the UUUC won 11 out of 12 parliamentary seats in Northern Ireland on a fiercely anti-Sunningdale platform, despite winning barely 50% of the popular vote. This result was a fatal blow for the Executive, which soon collapsed. Under West's leadership the party recruited Enoch Powell, who became Ulster Unionist MP for South Down. Powell advocated a policy of integration, whereby Northern Ireland would be administered as an integral part of the United Kingdom. This policy was to cause ruptions both within the Ulster Unionists and within wider Unionism as Powell's ideas conflicted with those committed to the restoration of devolved government to the province. The party also made gains when the Vanguard Party broke up and the rump merged back into the Ulster Unionists. The United Ulster Unionist Party (UUUP) emerged from the remains of Vanguard but folded in the early 1980s, as did the UPNI. In both cases the main beneficiaries of this were the Ulster Unionists, now under the leadership of James Molyneaux (1979-1995). In October 1995, Patricia Campbell, a Catholic member of the UUP and personal secretary to Molyneaux, filed an employment discrimination case against the UUP with the Fair Employment Commission which was settled out of court.

The party was led by David Trimble between 1995 and 2005. Although his support (which some nationalists claim to be ambiguous) for the Belfast Agreement caused a rupture within the Party into pro-agreement and anti-Agreement factions, he maintained unity for a while. Trimble served as First Minister of Northern Ireland in the power-sharing administration, created under the Belfast Agreement.

The UUP had a Roman Catholic Member of the Legislative Assembly (MLA) (the Northern Ireland Legislative Assembly), Sir John Gorman until the 2003 election. In March 2005, the Orange Order voted to end its official links with the UUP, while still maintaining the same unofficial links as other interest groups. Mr Trimble faced down Orange Order critics who tried to suspend him for his attendance at a Catholic funeral for a young boy murdered by the Real IRA, in the infamous Omagh bombing. Trimble and Irish president Mary McAleese, in a sign of unity, walked into the church together.