The Ulster Unionist Party (UUP, sometimes referred to as the Official Unionist Party or OUP) is the a political party in Northern Ireland representing the unionist community, and was the party of government in Northern Ireland between 1921 and 1972. Until 2003 it was the largest unionist party, and the largest party, but it has been overtaken in both ways by the Democratic Unionist Party.

It came into existence as the Irish Unionist Party in the 1880s to resist any granting of home rule to Ireland within the United Kingdom, which was the main demand of the Irish Parliamentary Party under leaders Isaac Butt, William Shaw, Charles Stewart Parnell and John Redmond. As with its nationalist counterparts, the party had a strong association with religion through the religious and political Orange Order, an institution which some compare to the Catholic Ancient Order of Hibernians. Though most unionist support was based in the geographic area that became Northern Ireland, its initial leadership all came from the south, with people like Colonel Saunderson, the Earl of Middleton 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 southern Irish Free State, many sitting in its senate or joining its political parties. Unionism's northern wing evolved into a separate Ulster Unionist Party.

Until almost the very end of its period of power in Northern Ireland the UUP was led by a combination of landed gentry (Sir Basil Brooke [later Lord Brookeborough] (1943-63), Terence O'Neill (1963-69) and James Chichester-Clark (1969-71) and gentrified industrial magnates (Sir James Craig [later Lord Craigavon] (1921-40), John Miller Andrews (1940-43)). Its last Prime Minister of Northern Ireland, Brian Faulkner (1971-72) was from the middle-class.

In 1922, Sir Edward Carson warned the new unionist leadership of Northern Ireland against practicing any discrimination towards the catholic minority in the province. It was advice that went unheeded. As current 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 UUP 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 the Ian Paisley, coupled with the heavy-handled behaviour of the British Army and the Royal Ulster Constabulary, led to a resurgence in violence by the Provisional IRA, a breakaway from the more marxist Official IRA and Official Sinn Féin. Faced with what seemed like a threat of civil war, the British Government ended the Party's hold on power in Northern Ireland, when it suspended the Stormont Parliament in March 1972.

Some liberal Unionists, who advocated the policies of Terence O'Neill left and joined the Alliance Party, while the emergence of Ian Paisley's Democratic Unionist Party (DUP) drew off working class and right-wing support. A more militant wing of the Party turned to the Vanguard movement to steer the Party back to its "traditional" course.

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 extents 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 estabished a separate branch in Northern Ireland which has had miniscule electoral success. There is frequent speculation that the Ulster Unionists may one day reunite with the Conservative Party.

With the failure of the power-sharing Executive set up the then Ulster Unionist leader, Brian Faulkner as Chief Executive, the Party split again with the resignation of Faulkner and his setting up of the Unionist Party of Northern Ireland (UPNI). Under the leadership of James Molyneaux (1979-1995) the Party profited from the demise of the UPNI, while surviving the challenge of the DUP.

The present Party is led by David Trimble (1995- present) and although his support (which some nationalists claim to be ambiguous) for the Belfast Agreement has caused a rupture within the Party into pro-agreement and anti-Agreement factions, he has so far maintained unity. Trimble has served as First Minister of Northern Ireland in a power-sharing administration, created under the Belfast Agreement. In a sign of the changing nature of modern Northern Ireland, the UUP now has some Roman Catholics MLAs (member of the Northern Ireland Legislative Assembly), while a debate is continuing on whether to break the link with the Orange Order. (Trimble faced down Orange Order critics who tried to suspend him for his attendance at a catholic funeral for a young boy murdered by another breakaway IRA, called the Real IRA, in the infamous Omagh bombings. Trimble and Irish president Mary McAleese, in a sign of peace, walked into the church hand in hand.)