The Afon Hydfer is a short river in Powys in the west of the Brecon Beacons National Park, south Wales. Its headwaters rise on the northeastern slopes of the Black Mountain as the streams named Trinant, Nant y Cadno, Nant yr Erydr and Crognant joining to flow down Cwm yr Afon on the western edge of Glasfynyd Forest. The river turns east at Blaenau Uchaf, then northeast through Cwm Hydfer and joins the River Usk just below Pont ar Hydfer.