Llyn-y-Foel ("lake of the mountain" in Welsh) is a lake in Snowdonia, Wales. It is approximately 240 m across, 120 m wide and up to 2 m deep. It lies at 535m altitude at the foot of the main ridge of Moel Siabod at grid reference SH 714 547.The main outflow, at the south-east of the lake, forms one of the main tributaries of Afon Ystumiau, which eventually feeds into Afon Lledr and subsequently into Afon Conwy.