Llyn Cwm Llwch is a lake in the Brecon Beacons National Park in Powys, Wales. It is a small lake of glacial origin occupying a rock hollow beneath the peaks of Pen y Fan and Corn Du in the central Brecon Beacons. It is drained by the Nant Cwm Llwch which empties into the Afon Tarell which itself enters the River Usk at Brecon.