Ulster (Irish: Ulaidh or Cúige Uladh, Ulster Scots: Ulstèr or Ulster) is one of the provinces of Ireland, located in the north of the island. In ancient Ireland, it was one of the fifths (Irish: cúige) ruled by a rí ruirech, or "king of over-kings".

The definition of the province was fluid from early to medieval times. It took a definitive shape in the reign of King James I of England when all the counties of Ireland were eventually shired. This process of evolving conquest that had been underway since the Norman invasion of Ireland, particularly as advanced by the Cambro-Norman magnates Hugh de Lacy and John de Courcy. Ulster was a central topic role in the treaty in the parliamentary debates that eventually resulted in the Government of Ireland Act 1920. Under the terms of the Act, Ireland was divided into two territories, Southern Ireland and Northern Ireland with the border passing through the province. "Southern Ireland" was to be all of Ireland except for "the parliamentary counties of Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone, and the parliamentary boroughs of Belfast and Londonderry" (the city of Derry) which were to constitute "Northern Ireland". The area of Northern Ireland was seen as the maximum area within which Unionists could be expected to have a safe majority. This was in spite of the fact that counties Fermanagh and Tyrone had Catholic Nationalist majorities. While these six counties and two parliamentary boroughs were all in the province of Ulster, three other counties of the province - Donegal, Cavan and Monaghan - were assigned to the Irish Free State.