The Monster Fish of Lake Shinji （宍道湖の怪魚） is a cryptid reported from Lake Shinji located in Shimane Prefecture, Japan.
In July 1972, a fisherman fishing in the lake noticed that the center of the lake was unnaturally bubbling. Then, the head of a huge fish emerged from it. He estimated the fish to be 5 meters (16.5 feet) long, although no detailed characteristics of the fish were mentioned.
Lake Shinji is a brackish lake connected to the ocean, and since shortfin mako sharks have been caught there in the past, it is believed to be one of these saltwater fish, but the species has not been identified.