Killough is a village and townland in County Down, Northern Ireland. It lies on the Irish Sea shore near Ardglass, five miles southeast of Downpatrick. It is a conservation area notable for its sycamore-lined main street. In the 2001 Census it had a population of 845 people.

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.