Merthyr Tydfil County Borough is a county borough in the south-east of Wales. In mid 2018, it had an estimated population of 60,183 making it the smallest local authority in Wales by both population and land area. It is located in the historic county of Glamorgan and takes its name from the town with the same name. The county borough consists of the northern part of the Taff Valley and the smaller neighbouring Taff Bargoed Valley. It borders the counties of Rhondda Cynon Taf to the west, Caerphilly County Borough to the east, and Powys to the north.

Until 1974, Glamorgan, or sometimes Glamorganshire, was an administrative county in the south of Wales, and later classed as one of the thirteen historic counties of Wales. Originally an early medieval petty kingdom of varying boundaries known in Welsh as Morgannwg, which was then invaded and taken over by the Normans as the Lordship of Glamorgan. The area that became known as Glamorgan was both a rural, pastoral area, and a conflict point between the Norman lords and the Welsh princes. It was defined by a large concentration of castles.