Llyn Ogwen is a lake in Conwy County Borough in north Wales. It lies alongside the A5 road between two mountain ranges, the Carneddau and the Glyderau.
Llyn Ogwen lies at a height of about 310 metres above sea level and has an area of 78 acres, but is a very shallow lake, with a maximum depth of only a little over 3 metres. It is fed by a number of streams from the slopes of the mountains which surround it, which include Tryfan and Pen yr Ole Wen. The largest of these streams is Afon Lloer, which flows from Ffynnon Lloer.
Llyn Ogwen is the source of the River Ogwen which flows north to reach the sea near Bangor. Although Ogwen is a natural lake, a dam was built in the early 20th century to raise the level of the lake in order to provide water for the use of the nearby Penrhyn Quarry. The lake is popular with anglers and is said to contain excellent trout.