County Antrim (named after the town of Antrim, from Irish Aontroim, meaning 'lone ridge') is one of the six counties of Northern Ireland, located within the historic province of Ulster. Adjoined to the north-east shore of Lough Neagh, the county covers an area of 3,086 square kilometres (1,192 sq mi) and has a population of 651,321, as of the 2021 census. County Antrim has a population density of 211 people per square kilometre or 546 people per square mile. It is also one of the thirty-two traditional counties of Ireland.

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

According to the 2001, 2011, and 2021 censuses it is currently one of only two counties of the island of Ireland in which a plurality or majority of the population are from a Protestant background. The other is County Down to the south.

A large portion of Antrim is hilly, especially in the east, where the highest elevations are attained. The range runs north and south, and, following this direction, the highest points are Knocklayd 514 m (1,690 ft), Slieveanorra 508 m (1,670 ft), Trostan 550 m (1,800 ft), Slemish 437 m (1,430 ft), Agnew's Hill 474 m (1,560 ft) and Divis 478 m (1,570 ft). The inland slope is gradual, but on the northern shore the range terminates in abrupt and almost perpendicular declivities, and here, consequently, some of the finest coast scenery in the world is found, widely differing, with its unbroken lines of cliffs, from the indented coast-line of the west. The most remarkable cliffs are those formed of perpendicular basaltic columns, extending for many miles, and most strikingly displayed in Fair Head and the celebrated Giant's Causeway. 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 Waterfoot on Red Bay, Carnlough and Glenarm, Larne on the Sea of Moyle, and Whitehead on Belfast Lough. All are somewhat exposed to the easterly winds prevalent in spring. The only island of size is the L-shaped Rathlin Island, off Ballycastle, 11 km (6.8 mi) in total length by 2 km (1.2 mi) maximum breadth, 7 km (4.3 mi) 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 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 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 4.5 m (15 ft) lower than Lough Neagh.

County Antrim has a number of air, rail, and sea links.

Northern Ireland's main airport, Belfast International Airport, at Aldergrove, is in County Antrim. Belfast International shares its runways with 38 Brigade Flying Station Aldergrove, which otherwise has its own facilities. It is the fifth-largest regional air cargo centre in the UK. There are regular services to Great Britain, Europe, and North America.

The region is also served by George Best Belfast City Airport, a mile east of Belfast city centre on the County Down side of the city, which was renamed in 2006 in honour of footballer George Best.