Fforest Fach is a small area of unenclosed land within Brecon Beacons National Park southwest of Sennybridge in the county of Powys, Wales. Translating from Welsh as 'little forest', it is named in opposition to the much larger area of unenclosed land to the south which is known as Fforest Fawr or the 'great forest'. Whilst Fforest Fawr and Fforest Fach have been distinct areas for many centuries, the odd situation now arises where Fforest Fach now finds itself within Fforest Fawr Geopark, the boundaries of which are drawn rather more widely than those of the traditional royal hunting forest.
The hill possesses two indistinct summits, the higher of which at 382m is in the south whilst that to the north at 381m is crowned by a trig point. There was formerly a rifle range at the north-western corner of the hill.