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.

From the eastern coast the hills rise instantly but less abruptly, and the indentations are wider and deeper. On both coasts there are several resort towns, including Portrush (with well-known golf links), Portballintrae and Ballycastle; on the east Cushendun, Cushendall and Milltown on Red Bay, Carnlough and Glenarm, Larne, and Whitehead on Belfast Lough. All are somewhat exposed to the easterly winds prevalent in spring. The only island of size is Rathlin Island, off Ballycastle, 6½ miles in length by 1½ in breadth, 7 miles from the coast, and of similar basaltic and limestone formation to that of the mainland. It is partially arable, and supports a small population. Islandmagee is in fact a peninsula separating Larne Lough from the North Channel.

The valleys of the Bann and Lagan, with the intervening shores of Lough Neagh, form the fertile lowlands. These two rivers, both rising in County Down, are the only ones of importance. The latter flows to Belfast Lough, the former drains Lough Neagh, which is fed by a number of smaller streams. The fisheries of the Bann and of Lough Neagh (especially for salmon and eels) are of value both commercially and to sportsmen, the small town of Toome, at the outflow of the river, being the centre. Immediately below this point lies Lough Beg, the "Small Lake," about 15 feet lower than Lough Neagh.