Langisjór is a lake in the Highlands of Iceland. It is around 20 km in length and up to 2 km wide, with a total surface area of about 26 km2 and a depth of 75m at its deepest point.

The lake is situated rather far from civilization at the south-western border of Vatnajökull at an altitude of 670 m above sea level. Environmental campaigners have expressed concern at government plans to site an industrial dam on the lake.