County Armagh is a county in Ulster. It is the smallest of the six counties that form Northern Ireland. County Armagh is known as the Orchard County because the land is so fertile for apple-growing. Its main town is Armagh, in the middle of the county, although Lurgan at the extreme north-east has a larger population.

The county borders Lough Neagh to the north, County Down to the east, County Tyrone to the north-west, and counties Louth and Monaghan, both in the Republic of Ireland, to the south and south-west respectively.

The River Blackwater runs along the border with County Tyrone. The River Bann enters Lough Neagh in the north, flowing though the north-east of the county. Mountains in Armagh include Slieve Gullion, Carrigatuke and Camlough Mountain as well as the Mega Mountain,which on a clear day day can be seen as far away as County Donegal.

There are also a number of islands in the county's section of Lough Neagh: Coney Island, Coney Island Flat, Croaghan Flat, Derrywarragh Island, Padian, Phil Roe's Flat and the Shallow Flat.

Ancient Armagh was the territory of the Ulaid (also known as Voluntii, Ultonians, Ulidians, Ulstermen) before the fourth century AD. They were ruled by the Red Branch. Nearby Armagh town was their capital, Emain Macha, the Navan Fort. The Red Branch play an important role in the Ulster Cycle, as well as the Cattle Raid of Cooley. However, they were eventually driven out of the area by the Three Collas, who invaded in the 4th century and held power until the 12th. The Clan Colla ruled the area known as Airghialla or Oriel for these 800 years.

The chief Irish septs of the county were descendants of the Collas, the O'Hanlons and MacCanns, and the Ui Neill, the O'Neills of Fews. Armagh was divided into several baronies: Armagh was held by the O'Rogans, Lower Fews was held by O'Neill of the Fews, and Upper Fews were under governance of the O'Larkins, who were later displaced by the MacCanns. Oneilland East was the territory of the O'Garveys, who were also displaced by the MacCanns. Oneilland West, like Oneilland East, was once O'Neill territory, until it was then held by the MacCanns, who were Lords of Clanbrassil. Upper and Lower Orior were O'Hanlon territory. Tiranny was ruled by Ronaghan. Miscellaneous tracts of land were ruled by O'Kelaghan.

Armagh was the seat of St. Patrick, and in Roman Catholic tradition, continues to be his see.