The Black Mountains are a group of hills in south-eastern Wales, and a small part of Herefordshire, England. They are the easternmost of the three groups of hills that are covered by the Brecon Beacons national park, and are frequently confused with the westernmost, which are collectively called the Black Mountain . (To confuse matters further, there is a peak in the Black Mountains called Black Mountain.) They may be roughly defined as those hills north of Abergavenny, south of Hay-on-Wye, east of the A479 road (the Rhiangoll valley) and west of, or on, the English border. Offa's Dyke long distance footpath runs along the border.

There are few villages in this area and one youth hostel, at Capel-y-ffin. Antiquities include Llanthony priory , Tretower castle and court, the fort Crug Hywel , and Castell Dinas , an 11th to 13th century castle.

The highest mountain in the group is Waun Fach; other Marilyns include Sugar Loaf, Ysgyryd Fawr , Mynydd Troed , Graig Syfyrddin , Allt yr Esgair , Myarth , Mynydd Llangorse , Bryn Arw , and Black Mountain.