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 the Snowdon Massif and Mynydd Mawr in the northern part of Snowdonia National Park. It has an area of 215 acres (0.87 km²), and is over 120 ft (37 m) deep.