The Nant Llwydiarth is a small river in Mid Wales. It flows from the southern flank of Mynydd Hendre-ddu down to Pont Cymerau, north-east of Aberllefenni. Here it joins the Nant Ceiswyn to form the Afon Dulas that flows south to the Afon Dyfi. There was an ancient bridge at Pont Cymerau.