County Londonderry or County Derry (Irish: Contae Dhoire) is one of the six counties of Northern Ireland in the province of Ulster in Ireland. It was named after its main town – and later city and administrative centre – Derry (Londonderry), which lies in the north-western corner of the county.

The highest point in the county is the summit of Sawel Mountain (678m) on the border with County Tyrone. Sawel is part of the Sperrin Mountains, which dominate the southern part of the county. To the east and west, the land falls into the valleys of the Bann and Foyle rivers respectively; in the south-east, the county touches the shore of Lough Neagh, which is the largest lake in Ireland; the north of the county is distinguished by the steep cliffs, dune systems and remarkable beaches of the Atlantic coast.

The county is home to a number of important buildings and landscapes: these include the excellently preserved seventeenth-century city walls of Derry; the fine National Trust-owned Plantation house and estate at Springhill; the Mussenden Temple with its spectacular views of the Atlantic; the dykes, artificial coastlines and internationally important bird sanctuaries on the eastern shore of Lough Foyle; and the visitor centre at Bellaghy Bawn, close to the childhood home of Nobel laureate Seamus Heaney. In the centre of the county are the old-growth deciduous forests at Banagher and Ness Wood, where the Burntollet River flows over the highest waterfalls in Northern Ireland.

The county flower is the Purple Saxifrage.