Kleifarvatn (Icelandic pronunciation: [ˈkʰleiːvarˌvahtn̥] ) is the largest lake on the Reykjanes Peninsula in Iceland, situated in the southern part of the peninsula. It is located on the fissure zone of the Mid-Atlantic Ridge. The lake has no visible water coming in or going out as most of its water comes and leaves underground. The lake can be reached via a track, and there are two areas with high temperature that can be found not far from it: Seltún/Krýsuvík and another to the east. The lake's greatest depth is 97 m. After the 2000 Iceland earthquakes, the lake began to diminish, and 20% of its surface has since disappeared.

The novel Kleifarvatn by Icelandic writer Arnaldur Indriðason was named after the lake.