Brecknockshire, Welsh Sir Frycheiniog, historic county, south-central Wales, named for Brychan, a 5th-century prince later known as Brycheiniog. Brecknockshire is mostly part of the present Powys county, although small areas in the south lie within the present Monmouthshire county and the county boroughs of Blaenau Gwent, Merthyr Tydfil, and Rhondda Cynon Taff.
Brecknockshire is a region of mountainous terrain and moorlands intersected by river valleys and rolling farmland. About one-half of the historic county lies within Brecon Beacons National Park. Several mountain peaks in the southern highlands reach elevations of more than 2,400 feet (700 metres). Pen y Fan, the highest peak in the park, stands 2,906 feet (866 metres) above sea level. Because of its location at the edge of the Welsh highlands along what became the English border, Brecknock has been historically a centre of conflict between the Welsh and various invading forces. The Romans built a large fort in what became central Brecknockshire, not far from the town of Brecon, and the Normans built castles at Brecon and Hay-on-Wye, in eastern Brecknockshire. The Welsh made their last stand for independence at Builth Wells in the north when Llywelyn ap Gruffudd rode through the town and was killed at nearby Cilmery in 1282. The Brecknock museum is located in the cathedral town of Brecon, which is also the historic county town (seat).