|Primary inflows||Afon Lloer, Tryfan, Pen yr Ole Wen|
|Primary outflows||River Ogwen|
|Basin countries||United Kingdom|
|Surface area||78 acres (32 ha)|
|Surface elevation||310 m|
Llyn Ogwen is a ribbon lake in north-west Wales. It lies alongside the A5 road between two mountain ranges of Snowdonia, the Carneddau and the Glyderau. Somewhat unusually, the county boundary at this point is drawn so that the lake itself lies in the county of Gwynedd, but all the surrounding land (excluding the outflow) lies in Conwy County Borough.
Llyn Ogwen lies at a height of about 310 metres above sea level and has an area of 78 acres (320,000 m2), 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 Afon 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.
- Geraint Roberts (1995) The Lakes of Eryri, Carreg Gwalch ISBN 0-86381-338-0