Umibōzu (海坊主, "sea bonze") is a spirit from Japanese folklore.
The Umibōzu is said to live in the ocean and capsize the ship of anyone who dares speak to it. This spirit's name, which combines the character for "sea" with the character of "Buddhist monk," is possibly related to the fact that the Umibōzu is said to have a large, round head, resembling the shaven heads of Buddhist monks. Alternatively they are demonic Yōkai (spectres) that appear to shipwreck victims and fishermen. They are believed to be drowned priests, and exhibit the shaven head and typically appears to be praying. It is usually reported as having a grey, cloud-like torso and serpentine limbs.
According to one story, if angered, they ask that the crews provide a barrel that it proceeds to fill with sea water to drown them. To avoid this disastrous fate, it is necessary to give him a bottomless barrel. This folktale is likely related to another Japanese tradition, which says that the souls of people who have no one to look after their graves take refuge at sea.