Breconshire is an inland border shire in southeast Wales, bounded on the north by Radnorshire, on the east by the English county of Hertfordshire, on the south by Glamorganshire, on the west by Carmarthenshire and on the northwest by Cardiganshire. The area is 742 square miles.

The surface of the shire is mountainous, the chief ranges being the Black Mountains in the southeast, the Brecon Beacons in the center, and the Mynydd Eppynt Hills in the north. The principal rivers are the Usk, Wye, and yrfon, and the upper courses of the Towy, Neath, Tawe, and Taff. The county forms part of the great South Wales coal field. There are a variety of remains from the Roman days. The capital is Brecon.

Breconshire is now a district within the county of Powys.