Ysgol Gyfun Cwm Rhymni is a Welsh-medium school situated in the village of Fleur-de-Lys in the Rhymney Valley. Cwm Rhymni was founded in 1981 with little more than 100 pupils and as of 2007 this number has grown to over 1,000.
The school's motto is 'Tua'r Goleuni' (Towards the Light) and its badge is a dragon on its hind-legs. The school is traditionally represented by the colours red and black.
Cwm Rhymni was originally situated on two sites, a Lower school for years 7 and 8 and an Upper school for years 9 to 13, situated in Aberbargoed and Bargoed respectively. By the late 1990s these buildings were in very poor condition, and after a long struggle the school was finally granted permission to build a new state of the art one-site building in Fleur-de-lys, which opened in 2002. The new building was funded through PFI. The original Headmaster of Cwm Rhymni was Mr Huw Thomas, who was succeeded by Mr Hefin Mathias in 1995. Mr Mathias was in turn succeeded upon his retirement in 2008 by Mr Owain ap Dafydd.
Cwm Rhymni has a very large catchment area stretching from Rhymney near the top of the valley to Caerphilly. The breadth of the catchment area is reflected by the names of the three main corridors in the school; Ebwy, Sirhywi and Rhymni; three adjoining South Wales Valleys that all feed some pupils into Cwm Rhymni. Prior to the establishment of Ysgol Gyfun Gwynllyw in 1988, Cwm Rhymni's catchment area also included the whole of the county of Gwent and pupils attended the school from as far afield as Newport and Chepstow.
The school's four houses are named after Welsh saints: Mabon (house colour: yellow), Cadog (green), Tudful (purple) and Sannan (blue). Tudful was a later addition to the original three houses as the school's numbers grew in the early 1990s.