Waun Fach is the highest mountain in the Black Mountains in south-eastern Wales. It is one of the three Marilyns over 600m that make up the range, the others being Black Mountain and Mynydd Troed. To the north Rhos Fawr and the Radnor Forest can be seen.
It is situated at the head of the Grwyne Fechan valley, above and to the west of the Grwyne Fawr reservoir . It has an undistinguished (and almost indistinguishable ) rounded summit . The nearby tops on the ridge, Pen Trumau and Pen y Gadair Fawr, although lower, are very much more recognisable.