The megamouth shark (Megachasma pelagios) is the only species in the family Megachasmidae. It is one of the rarest shark species, as only 14 specimens have been seen since its discovery in 1976 (of those 14, four were released back to the ocean). It has been caught in the Pacific, Atlantic and Indian Oceans.

The megamouth was named by the popular media after its discovery (actual scientific classification took place six years later). It does have an abnormally large mouth which is surrounded by rubbery lips. On each jaw, it has up to fifty rows of greatly reduced teeth. It is a planktivore, and relies on its reduced teeth and gill rakers to trap its prey as it filter feeds. It lives in the epipelagic zone of the ocean, and appears to migrate vertically, possibly following its preferred prey (krill). It has a maximum length of at least 550 centimeters, but may attain even greater sizes. It has a brown-black dorsal surface, and a white ventral surface.

There is some dispute as to its evolutionary origin. At first, it was proposed that it evolved from the same ancestor as the basking shark. Then, some biologists argued that it shared a common ancestry with the mako, great white and thresher sharks. However, more recent research suggests that it may be the most ancient of the shark species and evolved its unusual filter feeding independently of other species.

The only confirmed predator of the megamouth, interestingly, is the sperm whale. The thirteenth specimen identified (30 August 1998) was observed being attacked by three sperm whales, which swam off as the researchers approached. The shark appeared to have sustained minor injuries, and swam off after a short period recuperating near the surface.