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 west of Trelewis, from which it is separated by the Taff Bargoed river, and from Nelson in Caerphilly county borough and has a population of 6,252, increasing to 6,356 at 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.