Amemasu (アメマス) or Ō-amemasu (大アメマス) is a giant whale or fish-like creature from Ainu folklore in Japan.
Amemasu lives in Lake Mashu in Hokkaidō and capsizes boats, creates earthquakes and causes other disasters.
In one tale, the amemasu swallows a deer that has come down to the lake to drink, but the deer's antler tears open the great fish's belly and kills it. The amemasu's enormous corpse then blocks up the lake and puts it in danger of flooding. A god in the form of a bird warns the people in villages nearby.
The villagers upstream escape to higher ground, but the people downstream, not believing the bird, find the amemasu's body and drag it out of the lake, after which the water comes rushing out with such force that everything downriver is washed away. That area is now the flat Konsengen'ya plain.