The River Cynon is a river in South Wales and is one of the larger tributaries of the River Taff.
It arises on the flanks of the Brecon Beacons as a number of small streams arising from springs in the Old Red Sandstone. However the young river soon crosses into the Carboniferous coal measures as it enters first the village of Penderyn and then Hirwaun where it crosses under the Heads of the Valleys road passing close to the last working deep mine in Wales, Tower Colliery. The remainder of the valley is strongly marked by the signs of past industrialisation including colliery waste tips, the relics of a large smokeless fuel plant at Abercwmboi which includes the remnants of a tip that burned out of control for many years. The river passes in a south easterly direction through Aberdare, Mountain Ash, Penrhiwceiber before eventually meeting up with the River Taff at Abercynon.
The quality of the river has improved dramatically following the closure of the coal industry and the demolition of the smokeless fuel plant, but these closures had significant financial and social impacts on the communities in the valley and local priorities may be more focussed on social regeneration than environmental improvements to the river.