The Nedd Fechan is a river almost wholly within the county of Powys, Wales. 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 (11 km) to join with the Afon Mellte at Pontneddfechan, their combined waters continuing as the River Neath 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. Downstream of this confluence it forms the boundary between Powys to its east and Neath Port Talbot to its west.