Amemasu () or Ō-amemasu (大アメマス) is a giant whale- or fish-like creature from Ainu folklore. Some of the largest amemasu are said to live in Lake Mashū and Lake Shikotsu in Hokkaidō, with smaller ones inhabiting lakes throughout northern areas of Honshu. The amemasu are known for capsizing boats, creating earthquakes, and causing other natural disasters.
An amemasu is able to take on a human appearance, usually of a beautiful woman, in order to lure young men to their deaths. The skin of an amemasu is said to be cold and clammy, much like fish skin, which is how they can be identified when they are in human form.