Ulster (Irish: Ulaidh or Cúige Uladh, Ulster Scots: Ulstèr or Ulster) is one of the four provinces of Ireland, located in the north of the island. In ancient Ireland, it was one of the fifths ruled by a "king of over-kings" Irish: rí ruirech. In modern times, clusters of counties have been attributed to certain provinces but these clusters have no legal status. The province itself, while enjoying common usage and forming a strong part of local identity, has no official function for local government purposes.

Ulster is composed of nine counties. Six of these (almost 57% of the land area) make up Northern Ireland: Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone. The three remaining counties (about 43% of the land area) are in the Republic of Ireland: Cavan, Donegal and Monaghan.