Brecknockshire (Welsh: Sir Frycheiniog), also known as Breconshire, or the County of Brecon is one of thirteen historic counties of Wales, and a former administrative county.
The bulk of the historic county formed the borough of Brecknock in southern Powys from 1974 to 1996 under the Local Government Act 1972. The parishes of Penderyn and Vaynor went instead to the Cynon Valley and Merthyr Tydfil districts in Mid Glamorgan, whilst the urban district of Brynmawr and the parish of Llanelly from Crickhowell Rural District became part of Blaenau Gwent. Since 1996, Penderyn has formed part of the Rhondda Cynon Taff county borough and Llanelly has formed part of the Monmouthshire principal area. According to the 2001 census the shire had a population of 42,075.
Brecknockshire is bounded to the north by Radnorshire, to the east by Herefordshire and Monmouthshire, to the south by Monmouthshire and Glamorgan, and west by Carmarthenshire and Cardiganshire. Area 475,224 acres (1,923 kmē). The county is predominantly rural and mountainous. The Black Mountains occupy the southeast of the county, the Brecon Beacons the central region, Fforest Fawr the southwest and Mynydd Eppynt the north. Most of the Brecon Beacons National Park lies within the county. 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. Of the many waterfalls in the county, Henrhyd Falls are particularly spectacular. The main towns are Brecon, Brynmawr, Builth Wells, Hay-on-Wye, Llanwrtyd Wells and Ystradgynlais. The most important industries are agriculture, forestry and tourism. The county corresponds roughly to the combined territories of the former Kingdom of Brycheiniog and the Kingdom of Builth which were brought together to form this traditional county following the defeat of the last independent Welsh prince and the signing of the Statute of Rhuddlan in 1284.
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 and became a county under the same Shire model as seen in England under the Laws in Wales Acts 1535-1542.