In Slavic mythology, vodyanoy (Russian: водяной, IPA: [vədʲɪˈnoj]; lit. '[he] from the water' or 'watery') is a water spirit. In Czech and Slovak fairy tales, he is called vodník (or in Germanized form: Hastrman), and often referred to as Wassermann in German sources. In Ukrainian fairy tales, he is called “водяник“ (vodyanyk).

He may appear to be a naked man with a pot belly (and bald-headed) wearing a hat and belt of reeds and rushes, conflicting with other accounts ascribing him green hair and a long green beard. The varying look has been attributed in commentary to his shape-shifting ability. When angered, the vodyanoy breaks dams, washes down water mills, and drowns people and animals. Consequently, fishermen, millers, and also bee-keepers make sacrifices to appease him. The vodyanoy would sometimes drag people down to his underwater dwelling to serve him as slaves.

The vodník in Czechia or Slovakia were said to use colored ribbons (sometimes impersonating peddlers, but also tying them to grass, etc., as lures in the landscape) to attract humans near water in order to snatch them.

In Russia, the vodyanoy is sometimes called the dedushka vodyanoy (Дѣдушка-водяной, "Water-Grandfather") or vodyanik (водяник).

He is said to dwell in a slough (омут), kettle hole (Котловина), or a whirlpool of a river, pond or lake, and liked especially to live near a watermill. One that dwells in marshlands may be called a bolotnyanik (Болотняник).

His usual appearance is that of a naked old man with a fat paunch of a belly and swollen face according to the Russian folklore collector, but a later English commentary using similar phraseology insisted the creature was not nude but bald, and concatenates additional commentary from the Russian source which says he is seen naked but covered in slime (тина), wearing a high boyar hat [ru]) made of green "club-rush" (or other sedges) and a green belt of that same "grass".

He is also described as an old man with green hair and (long) green beard The green beard turns white with when the moon wanes, as the immortal Vodyanoy ages or rejuvenates with the phases of the moon.

Or, rather than wearing plant-based clothing, a different source states he is covered in weeds and slime, and is scaly-skinned in his true form. Or rather a figure of giant stature covered in grass and moss. Or be "quite black with enormous red eyes and a nose as long as a fisherman's boot". Or that he is human-faced, but has huge toes, paws instead of hands, long horns, a tail, and eyes that burn like red-hot coals.