Llyn Melynllyn (Welsh  for yellow lake) is a lake within the Carneddau range of mountains in Snowdonia, North Wales.

It lies at a height of just over , and has an area of some .

Cliffs rise steeply from its western edge, up to the summit of Foel Grach, and down from which most of its feeder streams flow.

A small dam was built at its northern end in 1887, but this was deliberately breached in 1970.  The lake acts as a reservoir for the Llandudno area.

Less than a kilometre to its north lies the larger Llyn Dulyn.

The outflow from the lake is called Afon Melynllyn, this stream flowing north-east to join Afon Dulyn, itself a tributary of the river Conwy.

References 
The Lakes of Eryri by Geraint Roberts, Gwasg Carreg Gwalch, 1985

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