Llyn Celyn is a large reservoir constructed between 1960 and 1965 in the valley of the River Tryweryn in North Wales. It measures roughly 2 miles long by a mile wide, and has a maximum depth of 140 ft. It has the capacity to hold 16,400 million gallons of water.

It was originally to be called Llyn Tryweryn Mawr, but in September 1964 Liverpool Corporation agreed to the name change following a letter by the Tryweryn Defence Committee