The Rhiangoll is a river in the Brecon Beacons National Park in southern Powys, south Wales. It rises on the western slopes of Waun Fach in the Black Mountains and drops steeply down to the west into the north-south valley through which the A479 runs between Talgarth and the hamlet of Tretower and through the village of Cwmdu. It is joined by various minor tributary streams on its left bank including the Ffinnant, Nant yr Ychen, Nant-y-fedw and Nantygarreg, all of which rise on the flanks of the ridge extending between Pen Trumau and Pen Gloch-y-pibwr.