The River Taff () is a large river in Wales. It rises as two rivers in the Brecon Beacons — the Taf Fechan (Little Taff) and the Taf Fawr (Big Taff) — before joining to form the Taff north of Merthyr Tydfil. The river supports a number of migratory fish, including salmon, sea trout, and eel.

Taf Fawr

The Taf Fawr rises below Corn Du, south-west of Pen y Fan and flows south through Beacons Reservoir, Cantref Reservoir and Llwyn-on Reservoir.

Tributaries

Nant Ffrwd and Taf Fechan

Underneath Cefn Viaduct, the Nant Ffrwd emerges into the river from a deep, narrow gorge. To the south of Cefn Coed-y-Cymer, the Taf Fechan and Taf Fawr merge at the wooded confluence that gives Cefn Coed-y-Cymer its name.

Nant Morlais and Nant Rhydycar

The Taff continues south through the centre of Merthyr Tydfil, where it is joined by the Nant Morlais which emerges at Abermorlais from a culvert in the east bank. Just south of Merthyr weir, the Nant Rhydycar joins.

Taf Bargoed

South of Merthyr, the Taff begins to meander its way between Pentrebach and Abercanaid and through Troedyrhiw, Merthyr Vale and Aberfan towards Quakers Yard. At Quakers Yard the river has cut a sharp incised meander, locally known as "Fiddler's Elbow". It is here that the third of the three Taffs, the Taff Bargoed joins the main river.

Cynon and Rhondda

At Abercynon, it is joined by the Cynon river, and at Pontypridd it is joined by the Rhondda river. From Pontypridd, it runs roughly south, through Taff's Well and Radyr, before arriving at the northern boundary of the city of Cardiff in Llandaff.

Mouth of the Taff

In Cardiff, the natural course of the river was changed during the 19th century; from Cardiff Castle it now follows an artificial riverbed west of where it previously flowed, now following a path through Bute Park and next to the Cardiff Arms Park and the former National Stadium, now the Millennium Stadium, and into Cardiff Bay, which has now become an artificial lake due to the construction of a barrage across the mouth of this river and the River Ely It then flows out into the Severn estuary.

Taffy as nickname

The term "Taffy", used as a nickname for people from Cardiff and by extension from anywhere in Wales, is sometimes thought to originate from the name of the river. It may be seen as slightly derogatory. Alternatively, however, the term may have originated from "Dafydd" (David) which a popular Welsh first name.