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.

[1] 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.

[2]