Llyn Fawr has an area of 9.8 hectares (24 acres) and lies south of Rhigos in the Town of Aberdare in the unitary authority of Rhondda Cynon Taf. It lies near the source of the river Rhondda Fawr, at an altitude of 368 metres, and is overlooked by the crags of Craig y Llyn. Originally a smaller lake, it was converted into a reservoir in the early 20th century.