Cefn Cul is a hill in the Fforest Fawr sector of the Brecon Beacons National Park in the county of Powys, south Wales. It is a broad north-northeast to south-southwest aligned ridge whose summit reaches a height of 562m above sea level. The hill is bounded to the west by the upper valley of the River Tawe though which the old Trecastle road runs whilst to the east is the pass of Bwlch Bryn-rhudd over which the A4067 road runs from the Cray valley south to that of the Tawe.
Cefn Cul is formed from sandstones and mudstones of the Senni Beds and Brownstones Formations of the Old Red Sandstone laid down during the Devonian period. Whilst the regional dip of the rock beds is to the south, locally they vary and a broad anticlinal structure coincides with the hill. It is within the area designated in 2005 as Fforest Fawr Geopark.