The Nedd Fechan is a river almost wholly within the county of Brecknockshire, Wales, currently administered as part of the unitary authority of Powys. It rises on the eastern slopes of Fan Gyhirych in the Fforest Fawr section of the Brecon Beacons National Park and flows south for 7 miles (12 km) to join with the Afon Mellte at Pontneddfechan, their combined waters continuing as the River Neath (Welsh Afon Nedd) to the sea near Swansea. The only significant tributary of the Nedd Fechan is the Afon Pyrddin which joins it at Pwll Du ar Byrddin ('the black pool of the Pyrddin'). Downstream of this confluence it forms the boundary between Brecknockshire to its east and Glamorgan to its west.