The River Ely (Welsh: Afon Elái) is in South Wales flowing generally southeast, from Tonyrefail to Cardiff.

After flowing through Tonyrefail and through the grounds of the Royal Glamorgan Hospital at Ynysmaerdy, it follows the A4119 road through the valley pass formed between Mynydd Garthmaelwg, to the west and Llantrisant, to the east.

Prior to the 1980s it had received large volumes of poorly treated or untreated sewage from the urban areas in the valleys.

Even after the construction of sewage treatment works at Miskin and Rhiwsaeson, the quality of the effluent continued to cause pollution until the late 1980s.

The recovery of the river since that time owes much to the regulatory effort of the NRA, and more lately the Environment Agency Wales, and to the massive capital investment made by Dŵr Cymru / Welsh Water.