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