A water demon is a type of demon that appears in the game, and one of the less powerful ones.

Like most major demons, water demons may summon other demons when they attack. At least 5/6 of the summons will be other water demons. All chaotic demons are also eligible, but those whose difficulty is greater than the current dungeon level are exponentially unlikely.[1] In practise, that mixes a few foocubi to the summons. There is a 1/260 chance, however, that they may summon Juiblex or Yeenoghu.

In the early stages of the game, a common way to summon a water demon is by quaffing or dipping from a fountain. These demons may either be hostile or friendly. A hostile demon will attack. A friendly demon grants a wish. A demon will be hostile with probability (80+DL)/100, where DL is your current dungeon level. If you have the Amulet of Yendor, a demon will always be hostile. Although water demons are relatively weak for demons, they are likely to wipe out most early-game characters. Players who cannot kill them quickly will probably find themselves fighting two or more.

A player may not polymorph into a water demon.