Yassie of Yamato River （大和川のヤッシー） is an aquatic mammal cryptid thought to occur in the Yamato River flowing through Osaka, Japan.
The timing of the occurrence of the rumor is not certain, but there were already rumors of strange creatures appearing in the Yamato River at the end of the 1990s, which gained momentum in the early 2000s.
It is said to appear only when the river swells due to heavy rains, and is said to eat the carcasses of pigs and human remains washed into the river. Photographs of the creature are said to have been taken, but these have not been made public. According to one witness, Yassie is about 1 to 3 meters long and swims in a mammalian fashion.
Because they are carnivorous aquatic mammals, images of creatures close to the ancestors of whales are often used when they are presented on websites.