In Slavic folklore, the Rusalka is a female entity, often malicious toward mankind and frequently associated with water. Counterparts in other parts of Europe are the French Melusine and the Germanic Nixie. Folklorists have proposed a variety of origins for the entity, including that they may originally stem from Slavic paganism, where they may have been seen as benevolent spirits. Rusalki (plural) appear in a variety of media in modern popular culture, particularly in Slavic language-speaking countries, where they frequently resemble the concept of the mermaid.

According to some, the original “Rusalka" was an appellation used by pagan Slavic peoples, who linked them with fertility and did not consider Rusalki evil. The motivations of a Rusalka vary depending on where she is living, a fact that may have its roots in the ancient pagan myths. In places where plant life is bountiful and crops grow well, such as in Ukraine and areas around the Danube River, the Rusalki are charming and playful. However, in harsher climates, the Rusalki are wild and wicked. These malevolent spirits would crawl out from the water in the middle of the night in order to ambush humans (especially men) who they would then drag, alive, back into the watery depths.

In 19th-century versions, a Rusalka is an unquiet, dangerous being who is no longer alive, associated with the unclean spirit. Young women, who either committed suicide by drowning due to an unhappy marriage or who were violently drowned against their will (especially after becoming pregnant with unwanted children), live out time on Earth as Rusalki. It is accounted by most stories that the soul of a young woman who had died in or near a river or a lake would come back to haunt that waterway. This undead Rusalka is not invariably malevolent, and would be allowed to die in peace if her death is avenged. However, her main purpose is to lure young men, seduced by either her looks or her voice, into the depths of said waterways where she would entangle their feet with her long red hair and submerge them. Her body would instantly become very slippery and not allow the victim to cling on to her body in order to reach the surface. She would then wait until the victim had drowned, or on some occasions, tickle them to death as she laughed. It is also believed, by a few accounts, that Rusalki can change their appearance to match the tastes of men they are about to seduce. That being said, a Rusalka is generally considered to represent universal beauty, therefore is highly feared yet respected in Slavic culture.