Treharris is a small town and community in the Taff Bargoed Valley in the south of Merthyr Tydfil County Borough, South Wales. It is located about 0.6 miles (1.0 km) west of Trelewis, from which it is separated by the Taff Bargoed river, and 0.9 miles (1.4 km) from Nelson in Caerphilly county borough and has a population of 6,356 from the 2011 Census. As a community, Treharris includes the villages of Quakers Yard and Edwardsville. Due to steepness and narrowness of both the Taff and Taff Bargoed valleys at Treharris several notable bridges and viaducts have been built in the area.

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.