Mosasaurus ("lizard of the Meuse River") was a genus of mosasaur, a carnivorous, aquatic lizard, somewhat resembling a flippered crocodile, with elongated heavy jaws. The genus lived in the Cretaceous period (Mesozoic era), around 70-65 millions years ago in the area of modern Western Europe. The name means "Meuse lizard", as it was found near the Meuse River (Latin Mosa + Greek sauros lizard). The genus Capelliniosuchus, once thought to be a metriorhynchid crocodylian, is a junior synonym of Mosasaurus.
It was found in 1764 near the Maastricht hill (where the Maastrichtian gets its name).