Llyn Llydaw (Welsh for 'Brittany lake') is a natural lake in Snowdonia National Park on the flanks of Snowdon, Wales' highest mountain.

This long thin lake has formed in a cwm about one-third of the way up the mountain.

It is one of the most visited lakes in the United Kingdom, in that many thousands of people every year visit Snowdon and many walk past this lake on the Miners' Track.

Llyn Llydaw is the largest of the three lakes on Snowdon's eastern flank.

The lake featured in Robson Green's Wild Swimming Adventure (ITV December 2009), chosen because it is claimed to be the coldest lake in Britain.