Lough Neagh, sometimes Loch Neagh, (/ˌlɒx ˈneɪ/) is a large freshwater lake in Northern Ireland. Its name comes from Irish: Loch nEathach, meaning "Lake of Eathach" (Irish: [ɫ̪ɔx ˈn̠ʲahax]).

With an area of 392 square kilometres (151 sq mi), it is the largest lake in the British Isles and is ranked 31st in the List of largest lakes of Europe. Located twenty miles (30 km) to the west of Belfast, it is approximately twenty miles (30 km) long and nine miles (15 km) wide. It is very shallow around the margins and the average depth in the main body of the lake is about 9 m (30 ft); although at its deepest the lough is about 25 metres (80 ft) deep.

Of the 4550 km² catchment area, around 9% lies in the Republic of Ireland and 91% in Northern Ireland; altogether 43% of the land area of Northern Ireland is drained into the lough, which itself flows out northwards to the sea via the River Bann. As one of its sources is the Upper Bann, the Lough can itself be considered a part of the Bann.

Towns and villages near the Lough include Craigavon, Antrim, Crumlin, Randalstown, Toomebridge, Ballyronan, Ballinderry, Moortown, Ardboe, Maghery, Lurgan and Magherafelt.