Mývatn (Icelandic pronunciation: ​[ˈmiːˌvahtn̥]) is a shallow lake situated in an area of active volcanism in the north of Iceland, not far from Krafla volcano. It has a high amount of biological activity. The lake and the surrounding wetlands provides a habitat for a number of waterbirds, especially ducks. The lake was created by a large basaltic lava eruption 2300 years ago, and the surrounding landscape is dominated by volcanic landforms, including lava pillars and rootless vents (pseudocraters). The effluent river Laxá [ˈlaksˌauː] is known for its rich fishing for brown trout and Atlantic salmon.

The name of the lake (Icelandic mý ("midge") and vatn ("lake"); "the lake of midges") comes from the large numbers of midges present in the summer.

The name Mývatn is sometimes used not only for the lake but the whole surrounding inhabited area. The river Laxá, the lake Mývatn and the surrounding wetlands are protected as a nature reserve (the Mývatn–Laxá Nature Conservation Area, which occupies 4,400 km² (440,000 ha)).

Since the year 2000, a marathon around the lake takes place in the summer.