The Black Mountains (Welsh: Y Mynyddoedd Duon) 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. The Offa's Dyke long-distance footpath runs along the border. The area is popular with hillwalkers and pony trekkers.
There are few villages in this area and one youth hostel, at Capel-y-ffin. The Skirrid Inn also has a wider reputation, being claimed as the oldest Public House in Wales.
Antiquities include Llanthony Priory, Tretower castle and court, the Iron Age hill fort Crug Hywel, and Castell Dinas, an 11th- to 13th-century castle.
The highest mountain in the group is Waun Fach (811 m); other Marilyns include the Sugar Loaf, Ysgyryd Fawr, Mynydd Troed, Graig Syfyrddin, Allt yr Esgair, Myarth, Mynydd Llangorse, Bryn Arw, and Black Mountain. Ysgyryd Fawr is also known as the Skirrid, Skyrrid or 'Holy Mountain'.