Foel Fraith is a hill in the Black Mountain in the county of Carmarthenshire, southwest Wales. It lies within the Brecon Beacons National Park and Fforest Fawr Geopark. Its plateau-like summit attains a height of 602 metres (1,975 ft) above sea level. To the north the subsidiary summit of Cefn y Cylchau reaches 556 feet (169 m), and to the southeast is the subsidiary summit of Carn Fadog or Cefn Carn Fadog, which reaches a height of 512 metres (1,680 ft).
The hill has a split personality with its northern slopes dominated by the succession of Carboniferous Limestone rocks and its southern slopes by the Twrch Sandstone of the overlying Marros Group, but also dating from the Carboniferous period. In common with the regional trend the rock strata dip moderately to the south though locally the beds overlying the limestone have foundered as this soluble rock has dissolved away over millennia. There is an extensive area of shakeholes across the entire hill.