The River Elwy (Afon Elwy in Welsh) is a river in north Wales and is a tributary of the River Clwyd. The source of the river is sometimes said to be on the northern flank of Moel Seisog, south-east of Llanrwst, at Ordnance Survey grid reference SH853593. However the river is only actually called the Elwy at the village of Llangernyw, where three rivers, Afon Cledwen, Afon Collen and Afon Gallen, meet to form the Elwy. It flows eastwards through Llanfair Talhaiarn and a few miles downstream from this village it is joined by a tributary, the River Aled (Afon Aled) which has its source in Llyn Aled.
After passing through Bont-newydd, the river turns northwards again and flows through St. Asaph (Llanelwy or "the church enclosure on the Elwy" in Welsh). It joins the River Clwyd about half way between St. Asaph and Rhuddlan, and the waters of the two rivers can often be seen flowing side by side for several miles.
A number of caves along the lower valley of the Elwy are of great archaeological interest and are considered one of the most important groups of Palaeolithic and later caves and rock shelters in Britain. In particular the cave at Pontnewydd contained remains of Neanderthal man and is the most north-westerly site at which Neanderthal remains have been found. The Elwy is also well-known for its sea-trout fishing, and also has a small run of Atlantic Salmon.