County Antrim is one of the six counties that form Northern Ireland. It is the 9th largest of the 32 traditional counties of Ireland in terms of area, and 2nd in terms of population behind Dublin. It is situated in the north-east of the island of Ireland, in the province of Ulster. It is bounded north and east by the narrow seas separating Northern Ireland from Scotland, the Atlantic Ocean and Irish Sea, south by Belfast Lough and the River Lagan dividing it from County Down, south-west by Lough Neagh, dividing it from County Armagh and County Tyrone, and west by County Londonderry, the boundary with which is the River Bann.

The Glens of Antrim offer isolated rugged landscapes, the Giant's Causeway is a unique landscape and a UNESCO World Heritage site, Bushmills produces legendary whiskey, and Portrush is a popular nightlife zone. The majority of the capital city of Northern Ireland, Belfast, is also in County Antrim, with the remainder being in County Down.

At what date the county of Antrim was formed is not known, but it appears that a certain district bore this name before the reign of Edward II (early 14th century), and when the shiring of Ulster was undertaken by Sir John Perrot in the 16th century, Antrim and Down were already recognized divisions, in contradistinction to the remainder of the province. The earliest known inhabitants were of Celtic origin, and the names of the townlands or subdivisions, supposed to have been made in the 13th century, are all of Gaelic derivation. Antrim was exposed to the inroads of the Danes, and also of the northern Scots, who ultimately effected permanent settlements. In ancient times, it was inhabited by a Celtic people called the Darini. In the early Middle Ages, southern County Antrim was part of the Kingdom of Ulidia, ruled by the Dál Fiatach clans O'Haughey/O'Hoey and MacDonlevy/McDunlavey; the north was part of Dal Riada, which stretched into western Scotland over the Irish Sea. Dal Riada was ruled by the O'Lynch clan, who were vassals of the Ulidians. Besides the Ulidians and Dal Riada, there were the Dal nAraide of lower County Antrim, and the Cruithne, who were not Gaelic Celts but Picts. In the late Middle Ages, it was divided into three parts: northern Clandeboy, the Glynnes and the Route. The Cambro-Norman MacQuillans were powerful in the Route. A branch of the O'Neills of Tyrone migrated to Clandeboy in the 1300s, and ruled it for a time. Their family was called O'Neill Clannaboy. A galloglass sept, the MacDonnells, became the most powerful in the Glynnes in the 1400s.

Antrim is divided into 16 baronies. Lower Antrim, part of Lower Clandeboy, was settled by the sept O'Flynn/O'Lynn. Upper Antrim, part of Lower Clandeboy, was the home of the O'Keevans. Belfast was part of Lower Clandeboy and was held by the O'Neill-Clannaboys. Lower Belfast, Upper Belfast, and Carrickfergus were also part of Lower Clandeboy. Cary was part of the Glynnes; ruled originally by the O'Quinn sept, the MacDonnell galloglasses from Scotland took power here in the late Middle Ages and some of the O'Haras also migrated from Connaught. Upper and Lower Dunluce were part of the Route, and were ruled by the MacQuillans. Upper and Lower Glenarm was ruled by the O'Flynn/O'Lynn sept, considered part of the Glynns. In addition to that sept and that of O'Quinn, both of which were native, the Scottish gallowglass septs of MacKeown, MacAlister, and MacGee, are found there. Kilconway was originally O'Flynn/O'Lynn territory, but was held by the MacQuillans as part of the Route, and later by the gallowglass sept of MacNeill. Lower Massereene was part of Lower Clandeboy and was ruled by the O'Flynns and the O'Heircs. Upper Massereene was part of Lower Clandeboy, ruled by the O'Heircs. Upper and Lower Toome, part of the Route, were O'Flynn/O'Lynn territory. Misc was first ruled by the MacQuillans. Later, the Scottish gallowglass MacDonnells and MacAlisters invaded. The MacDonnells were a branch of the Scottish Clan MacDonald; the MacAlisters traced their origin back to the Irish Colla Uais, eldest of the Three Collas. Islandmagee had, besides antiquarian remains, a notoriety as a home of witchcraft, and was the scene of an act of reprisal against the Catholic population during the Irish Rebellion of 1641 for the massacre of Protestants, by the Scottish Covenanter soldiery of Carrickfergus.