Penwyllt (Welsh: "wild headland"[1]) is a hamlet located in the upper Swansea Valley in Powys, Wales, lying within the Brecon Beacons National Park.
A former quarrying village, quicklime and silica brick production centre, its fortunes rose and fell as a result of the Industrial Revolution in South Wales. It is now an important caving centre. It is in the community of Tawe-Uchaf.
Beneath Penwyllt and the surrounding area is the extensive limestone cave system of Ogof Ffynnon Ddu, part of which was the first designated underground national nature reserve in the UK. A corresponding area on the surface is also part of the national nature reserve, on the slopes of Carreg Cadno.