Langisjór (Icelandic pronunciation: [ˈlauɲcɪˌsjouːr̥] ) is a lake in the western part of Vatnajökull National Park, Iceland. It is around 20 km in length and up to 2 km wide, with a total surface area of about 26 km (10 sq mi) and a depth of 75m at its deepest point.

The lake is situated rather far from civilisation 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.

• List of lakes of Iceland

• Official Website of Vatnajökull National Park
• Photos: http://isafold.de/strutsstigur02/img_voetn.htm