Llyn Dulyn () is a lake on the edge of the Carneddau range of mountains in Snowdonia, North Wales. The lake is in extent and 189 feet (58 metres) deep. Less than a kilometre to its south lies the smaller Llyn Melynllyn. Cliffs rise steeply from the lake edge up to the summits of Garnedd Uchaf and Foel Grach, giving it a dark brooding appearance - hence its name.
