Londonderry is a maritime county in Northern Ireland, Ulster Province. It is bounded on the north by the Atlantic Ocean and Lough Foyle, on the east by County Antrim, on the south and southwest by Tyrone, and on the west by County Donegal in the Irish Republic. Its area is 816 square miles.
The surface of the county in the south and central sections is mountainous, the highest point being Mount Sawell of the Sperrin Range at 2236 feet. There are lowlands along Lough Neagh and along the northern coast. Elsewhere through the county are many fertile valleys.
The principal rivers are the Foyle, Roe, Moyola, Faugh, and Bann. The most important towns are Londonderry, the county seat; Coleraine, a Bann River port; Magherafelt; Ballymoney; and Castledawson. The ancient castle of Dunluce is at Portrush on the north coast.