The River Rhondda (Welsh: Afon Rhondda) is a river in south-east Wales which has two major tributaries; the Rhondda Fawr (English: Large Rhondda) and the Rhondda Fach (English: Little Rhondda).

Despite their names, both tributaries are about the same length. Both valleys are typical U shaped glaciated valleys in carboniferous sandstones and coal measures. The whole form of the river and its surrounding urbanisation has been dominated by coal and the mining communities that grew up to exploit the rich coal seams. Much of the valley has suffered severe subsidence because of the removal of coal from underneath the valley floor. The houses and streets have subsided with the result that river levels are, in parts , higher than the surrounding houses. In order to contain the river and prevent flooding, walls have been built, sometimes across the ends of streets - these are known as The Rhondda walls

The Rhondda Fawr rises near Llyn Fawr - and runs down the Rhondda Valley, (Welsh: Cwm Rhondda) to its confluence with the River Taff at Pontypridd.

The river runs through Blaenrhondda where it is joined by the Nant y Gwair via a classical example of a hanging valley. The river then passes through a string of mining towns and villages including Treherbert, Treorchy, Pentre, Ton Pentre, Ystrad Rhondda, Llwynypia, Tonypandy, Dinas, Porth and Trehafod before finally joining the River Taff in Pontypridd.

The Rhondda Fach has its source very close to the Rhondda Fawr on the hills above Blaenrhondda. The fledgling river is first contained in the Lluestwen reservoir before flowing down into Maerdy and then on through Ferndale, Tylorstown, and Ynyshir before joing its sister tributary at Porth.

The mining industry had a catastrophic impact on the quality of the river with all the mine waters being pumped straight into the river with no treatment. For very long periods, probably more than a century, the river was continuously black with coal solids and little if anything could live in the river. This was compounded by the very basic sewage disposal arrangements which saw all the sewage discharged into the same river. Not until the 1970s was there real investment made in improving the sewage treatment arrangements.

Since the early 1970s the river has been steadily improving in quality largely due to the closure of all the coal mines and through the investment in sewerage and sewage treatment.