Corn Du is a summit of the twin topped Pen y Fan and the second highest peak in South Wales at 871.5 m (2,864 ft), situated in the Brecon Beacons National Park.

The summit itself is marked by a well structured Bronze Age cairn with a central burial cist like that on nearby Pen y Fan.

It offers good views down into Cwm Llwch and across the Usk valley to Brecon as well as east towards the Sugar Loaf, Monmouthshire above Abergavenny.

All of the surrounding moorland is open to access, but crossing some parts of the terrain such as peat bogs is more difficult and the main paths are well-engineered and maintained by the National Trust.

The same Plateau Beds recur in the Black Mountain (range) to the west, and cap the summits of Picws Du and Fan Foel, as well as forming steep cliffs below the peaks.