The Annalong River is a mountain stream flowing from the Mourne Mountains in County Down, Northern Ireland, descending through rugged upland terrain before emptying into the Irish Sea at the small coastal village of Annalong, approximately 8 miles south of Newcastle. Draining one of Northern Ireland's most iconic mountain ranges, the river is a freestone system fed by rainfall and upland runoff, giving it the classic characteristics of a spate river — responsive to precipitation, with flows that can rise and fall quickly depending on weather conditions in the Mournes. The river passes through varied terrain before reaching the coast, where it exits through the outer channel of Annalong Harbour into the sea.