Cefn Cul is a hill in the Fforest Fawr sector of the Brecon Beacons National Park (Parc Cenedlaethol Bannau Brycheiniog) in Powys, southern Wales. It is a broad north-northeast to south-southwest aligned ridge whose summit reaches a height of 562 m 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 Crai valley south to that of the Tawe.