Kleifarvatn () 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. A 2012 survey demonstrated one definite geothermal vent under the lake and suggested two others in a linear line trending as for most of the faults in the area.