The River Derg is a small river in Ireland. The river has its source in Lough Derg, in County Donegal, Republic of Ireland, and it flows into County Tyrone, Northern Ireland, east through Castlederg to join with the River Strule to form the River Mourne. The upper reaches of the catchment are characterised by peatland, while the lower reaches flow predominantly through farmland. The River Derg\'s length is 45 km.
The River Derg is known for its salmon, grilse and sea trout.
