The River Duff, or Bunduff, (Irish: An Dubh, meaning "the black") is a river in County Sligo and County Leitrim, Ireland. The River Duff is 14 miles (23 km) long, flowing from Glenade Valley in County Leitrim to the sea. In County Leitrim it is also known as the Black River. It is joined by the Ballanaghtrillick River, which runs out of the Horseshoe Pass.