Glaslyn (Welsh: Blue lake) is a lake in the Snowdonia National Park in Gwynedd, north-west Wales.
It is found at approximately 600 metres (2,000 ft) above sea level in a cwm on the eastern flanks of Snowdon. It is a rough ellipse in shape, about 500 metres (1,600 ft) east–west and about 300 metres (980 ft) north–south, and the Miners' Track runs along the northern shore.
Glaslyn is the source of the Afon Glaslyn, the major river of Gwynedd, which runs east to the reservoir of Llyn Llydaw before turning south-west to reach the sea at Porthmadog.