Pen Allt-mawr is a 719 metres (2,359 ft) high subsidiary summit of Waun Fach and the third highest peak in the Black Mountains in south-eastern Wales.

A very recognisable and prominent peak of the Black Mountains, it lies near the end of the more westerly of Waun Fach's two broad southern ridges.

These rocks which date from the Late Devonian Epoch are continuous with those of Pen Cerrig-calch but other than a tiny outlier forming the summit of Sugar Loaf, are found nowhere else within the Black Mountains.

Beneath the Quartz Conglomerate are the Early Devonian sandstones of the Brownstones Formation which form the upper slopes and summits of Pen Twyn Glas and Mynydd Llysiau to the north (though are barely seen).

More readily eroded mudstone bands within this formation give rise to the stepped appearance of this hill and many of its neighbours.