Megathura crenulata is a northeast Pacific species of limpet in the family Fissurellidae known commonly as the great keyhole limpet or giant keyhole limpet. Megathura is a monotypic genus, in other words, this is the only species in that genus. This species occurs along the rocky coast of western North America, its distribution extending from Southern California to the Baja California peninsula in Mexico. It is found in the intertidal zone and in the sea up to a depth of 33 meters.