Llyn Dulyn (Welsh: Black lake) is a lake on the edge of the Carneddau range of mountains in Snowdonia, North Wales.
The lake covers an area of only 33 acres, yet is extremely deep - it reaches to a depth of 189'. 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. A number of superstitions are connected with the cwm, which has also been the site of a number of small aeroplane crashes.
The lake has a dam, originally built in 1881, to increase its water capacity, but it has been altered several times since. As a reservoir Dulyn (along with Melynllyn) provides water for the town of Llandudno. The pipes cross the River Conwy at Talycafn bridge.
The outflow from the lake is called Afon Dulyn, and this stream also feeds water into neighbouring Llyn Eigiau. Afon Dulyn flows north-east, passing Tal-y-bont before joining the River Conwy.