Mynydd Llangatwg or Llangattock Mountain is a hill in the Brecon Beacons National Park in the county of Powys, south Wales. It is named from the village of Llangatwg (or 'Llangattock') which sits in the valley of the River Usk to the north of it. It is essentially an undulating plateau rising in the west to a height of 530m at OS grid ref SO 171157 and in the east to a height of 529m at Hen Dy-aderyn / Twr Pen-cyrn. This spot is marked by a trig point. The hill forms an impressive northern scarp overlooking the Usk valley and commonly referred to as the Llangattock Escarpment. Its southern margins are more subdued. Its eastern end is defined by the drops into the Clydach Gorge. Beyond the B4560 to the west the hill merges with Mynydd Llangynidr which has a similar character.
Particular features of note include 'The Lonely Shepherd', an isolated limestone pinnacle which stands at the eastern tip of the plateau, left there by quarryworkers who removed great quantities of the surrounding rock. A number of cairns are scattered across the hill, notably the sizeable pair which decorate the summit of Twr Pen-cyrn and which are thought to be of Neolithic age. A more recent addition to the landscape was Cairn-Mound Reservoir which once impounded the headwaters of Nant yr Hafod on the southern slopes though Welsh Water abandoned this some years ago and its bed has revegetated, though the embankment remains. A couple of gas piepelines have been laid across the mountain and their courses can be traced variously by fences, vegetation changes and marker poles.