The River Ely (Welsh: Afon Elái) is in South Wales flowing generally southeast, from Tonyrefail to Cardiff.
The river is about 24 miles (39 km) long. The Ely's numerous sources lie in the mountains to the south of Tonypandy, near the town of Tonyrefail, rising in the eastern slopes of Mynydd Penygraig (Penygraig) and Mynydd y Gilfach (Penrhiwfer) and the western slopes of Mynydd Dinas (Williamstown), whose eastern slopes feed the Rhondda by Porth. The source of the Ogwr Fach lies just to the west.
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. Flowing through Talbot Green, the Ely is joined by the Afon Clun at Pontyclun. Although numerous smaller streams join the river, the Clun is its only large tributary.
The Ely turns to the southeast on reaching the Vale of Glamorgan alongside the South Wales Main Line immediately south of Miskin and after being traversed by the M4 motorway, it flows through farmland. It passes the St Fagans National History Museum and continues towards Cardiff and Ely (Welsh: Trelái) before flowing under the Vale of Glamorgan Line and into Cardiff Bay at Penarth Marina.
The Ely Subway is a closed pedestrian tunnel under the mouth of the Ely. The tunnel opened in 1900 replacing a chain ferry, and was used by workers at Cardiff Docks and Penarth Dock. It was closed in 1965.