Brecknockshire (Welsh: Sir Frycheiniog), also known as the County of Brecknock, Breconshire, or the County of Brecon is one of thirteen historic counties of Wales, and a former administrative county.

Brecknockshire was bounded to the north by Radnorshire, to the east by Herefordshire and Monmouthshire, to the south by Monmouthshire and Glamorgan, and to the west by Carmarthenshire and Cardiganshire. The county is predominantly rural and mountainous. The Black Mountains occupy the southeast of the area, the Brecon Beacons the central region, Fforest Fawr the southwest and Mynydd Epynt the north. The highest point is Pen y Fan, 2907 ft (886 m). The River Wye traces nearly the whole of the northern boundary, and the Usk flows in an easterly direction through the central valley. The main towns are Brecon, Builth Wells, Crickhowell, Hay-on-Wye, Llanwrtyd Wells, Talgarth and Ystradgynlais.

The kingdom of Brycheiniog was established in the 5th century and survived until the 10th century when it was subjugated by the Anglo-Saxons. During the Norman period, the area was classified as a Lordship. The Lord of Brycheiniog was subject to the Mortimer family who ruled most of south and east Wales in an area called the Welsh Marches. During the reign of Llywelyn ap Gruffydd of Gwynedd the homage of the Lord of Brycheiniog was transferred to him from the King of England (Henry III) by the Treaty of Montgomery in 1267. However, it was an attack on Brycheiniog by the Marcher Lords Humphrey de Bohun and Roger Mortimer in 1276 which led to the final breakdown of the peace between England and Wales after which Llywelyn's domain was reduced to just his lands in Gwynedd. Brycheiniog was thereafter subject to the King of England.