The River Blackwater or Ulster Blackwater is a river mainly in County Armagh and County Tyrone, Northern Ireland. Its source is to the north of Fivemiletown, County Tyrone. The river divides County Armagh from County Tyrone and also divides County Tyrone from County Monaghan, making it part of the border between the United Kingdom and the Republic of Ireland.
The Blackwater\'s length is 91.3 km. If the Blackwater\'s flow is measured through its path through the 30 km Lough Neagh and onwards to the sea via the 64.4 km Lower Bann, the total length is 186.3 km. This makes the Blackwater–Neagh–Bann the longest natural stream flow in Ulster and is longer than the Munster Blackwater.
