Pen y Gadair Fawr is an 800 metres (2,625 ft) high subsidiary summit of Waun Fach and the second highest peak in the Black Mountains in south-eastern Wales.

Marked by a medium-sized cairn, it is a much more distinguished top than its parent 1.5 km to the northwest.

Its 658 metres (2,159 ft) high top Pen Twyn Mawr is about 2 km to the southeast.

Mudstone layers within these sandstones are more readily eroded and have given rise to the stepped appearance of parts of the mountain, not least the summit section.

Peat has accumulated on parts of the hill in the postglacial period, notably north towards Waun Fach.