Kunashiri Island Sea Monster is a type of sea serpent reported from Kunashiri Island in the Northern Territories of Japan.
Japanese cryptid researcher Bintaro Yamaguchi received a report of a cryptid from a person. The person's father had often seen the monster while fishing in the sea near Kunashiri Island during the period before World War II. It was black in color and looked like a whale, but had distinctly different fins and a long neck. It was not ferocious, and often several of them swam alongside the fishing boats.
The reported features appear to be like those of a plesiosaur or nothosaur, but the time and environment makes it unlikely that these creatures survived. Perhaps it was a hoax or a type of seal.