Llyn Elsi is a lake located above the village of Betws-y-coed in the Snowdonia National Park in North Wales. It is a reservoir providing water for the village.
There is a path round the lake, and at a height of over 700' it affords good views to the north-west towards the mountain ranges of the Carneddau and the Glyderau. Snowdon itself is not visible behind the mass of Moel Siabod.
The 20' dam was built in 1914, but even so the lake only has a maximum depth of some 31'. Prior to this the lake was in fact two smaller lakes, called Llyn Rhisgog and Llyn Enoc. The lake has a small island.
A memorial at the northern end of the lake commemorates the fact that Lord Ancaster permitted the building of the dam and the use of the lake as a water supply.
The lake is kept stocked with various type of trout by the local angling club.
The lake can be reached on foot by various paths, the most direct being that which starts behind St. Mary's church in the village.