Jökulsárlón is a large glacial lake in southern part of Vatnajökull National Park, Iceland. Situated at the head of the Breiðamerkurjökull glacier, it developed into a lake after the glacier started receding from the edge of the Atlantic Ocean. The lake has grown since then at varying rates because of melting of the glaciers. The glacial front is now about 8 km (5.0 mi) away from the ocean's edge and the lake covers an area of about 18 km2 (6.9 sq mi). In 2009 it was reported to be the deepest lake in Iceland, at over 284 m (932 ft), as glacial retreat extended its boundaries. The size of the lake has increased fourfold since the 1970s.

A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier.