Pen Twyn Mawr is a top of Pen y Gadair Fawr in the Black Mountains in south-eastern Wales. It lies on one of the many south ridges of Waun Fach.

The summit is marked by a pile of stones in an area of sandy soil and heather. Pen y Gadair Fawr is to the north, Chwarel y Fan to the east and Pen Allt-mawr to the west.

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51°57′47″N 3°08′38″W/51.96292°N 3.14392°W/ 51.96292; -3.14392