Cwm Prysor Viaduct, which is occasionally referred to as Blaen-y-Cwm Viaduct, is a railway viaduct which crosses the Afon Prysor in thinly populated uplands east of Trawsfynydd, Gwynedd, Wales. It was built by the Bala and Festiniog Railway. It carried a singly track on a line that ran between Bala Junction and Blaenau Ffestiniog. The line it was built for went out of service in 1961.