The River Colwyn ( Afon Colwyn or Nant Colwyn in Welsh) is a small river in North Wales, a tributary of the River Glaslyn.
It has its source on the south-western flank of Snowdon. It first flows west to cross the A4085 Caernarfon to Beddgelert road at Pont Cae'r Gors just south of Rhyd Ddu, then flows south parallel to the A4085 to Beddgelert. On the way it is joined by two tributaries, Afon Cwm-du and Afon Meillionen. At Beddgelert it joins the Glaslyn, to then flow south to Tremadog Bay near Porthmadog.
The Colwyn is a short and fast-flowing river. It generally carries less water than the Glaslyn, but water levels can rise very rapidly when there is heavy rain. The river is popular with canoeists and anglers.