A megatsunami is a term used for a very large wave created by a large, sudden displacement of material into a body of water.

Megatsunamis have quite different features from other, more usual type of tsunamis. Most tsunamis are caused by underwater tectonic activity (movement of the earth's plates) and therefore occur along plate boundaries and as a result of earthquake and rise or fall in the sea floor, causing water to be displaced. Ordinary tsunamis have shallow waves out at sea, and the water piles up to a wave height of up to about 10 metres (33 feet) as the sea floor becomes shallow near land. By contrast, megatsunamis can occur in locations where there is a very large amount of material that suddenly falls into water, or anywhere in water (for meteor impact), or may be caused by volcanic activity, and can have extremely high initial wave heights of hundreds and possibly thousands of metres, far beyond any ordinary tsunami, as the water is "splashed" upwards and outwards by the impact or displacement. As a result, two heights are sometimes quoted for megatsunamis – the height of the wave itself (in water), and the height to which it surges when it reaches land, which depending upon the locale, can be several times larger.