Strangford Lough is a large sea lough or inlet in County Down, in the east of Northern Ireland. It is the largest inlet in Ireland and the wider British Isles, covering 150 km2 (58 sq mi). The lough is almost fully enclosed by the Ards Peninsula and is linked to the Irish Sea by a long narrow channel at its southeastern edge. The main body of the lough has at least seventy islands along with many islets (pladdies), bays, coves, headlands and mudflats. It is part of the Strangford and Lecale Area of Outstanding Natural Beauty. Strangford Lough was designated as Northern Ireland's first Marine Conservation Zone in 2013, and has been designated a Special Area of Conservation for its important wildlife.

County Down is one of the six counties of Northern Ireland, one of the nine counties of Ulster and one of the traditional thirty-two counties of Ireland. It covers an area of 961 sq mi (2,490 km2) and has a population of 552,261. It borders County Antrim to the north, the Irish Sea to the east, County Armagh to the west, and County Louth across Carlingford Lough to the southwest.