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.

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.