The Ulster Unionist Party (UUP) is a unionist political party in Northern Ireland. The UUP is the third oldest political party in the United Kingdom, and the oldest political party on the island of Ireland. The party was founded as the Ulster Unionist Council in 1905, emerging from the Irish Unionist Alliance in Ulster. Under Sir Edward Carson, it led unionist opposition to the Irish Home Rule movement. Following the partition of Ireland, it was the governing party of Northern Ireland between 1921 and 1972. It was supported by most unionist voters throughout the conflict known as the Troubles, during which time it was often referred to as the Official Unionist Party (OUP).
Under David Trimble, the party helped negotiate the Good Friday Agreement of 1998, which ended the conflict. Trimble served as the first First Minister of Northern Ireland from 1998 to 2002. However, it was overtaken as the largest unionist party in 2003 by the Democratic Unionist Party (DUP). As of 2022 it is the fourth-largest party in the Northern Ireland Assembly, after Sinn Féin, the DUP, and the Alliance Party. Since August 2024 the party has been led by Mike Nesbitt.