Llyn Cwellyn (Llyn Quellyn in some antiquated texts) is a reservoir in North Wales which supplies drinking water to parts of Gwynedd and Anglesey. It lies on the Afon Gwyrfai in Nant y Betws between Moel Eilio and Mynydd Mawr in the northern part of Snowdonia National Park. It has an area of , and is over deep. At the southern end is the small village of Rhyd Ddu.