Kappa ("river-child"), alternately called Kawatarō , river-boy") or Kawako , "river-child"), are legendary creatures, a type of water sprite found in Japanese folklore. In Shintō they are considered to be one of many suijin. Most depictions show kappa as child-sized humanoids, though their bodies are often more like those of monkeys or frogs than human beings. Some descriptions say their faces are apelike, while others show them with beaked visages more like those of tortoises or with duck beaks. Pictures usually show kappa with thick shells and scaly skin that ranges in color from green to yellow or blue.

Kappas supposedly inhabit the ponds and rivers of Japan and have various features to aid them in this environment, such as webbed hands and feet. The most notable feature of the kappa, however, is the water-filled depressions atop their heads. These cavities are surrounded by scraggly hair, and this type of bobbed hairstyle is named okappa-atama for the creatures. The kappa derive their incredible strength from these liquid-filled holes, and anyone confronted with one may exploit this weakness by simply getting the kappa to spill the water from its head. The kappa possesses a deep sense of etiquette, so one trusted method is to appeal to this, for a kappa cannot help but return a deep bow, even if it means losing its head-water in the process. Once depleted, the kappa is seriously weakened and may even die. Other tales say that this water allows kappa to move about on land, and once emptied, the creatures are immobilized.