Jenny Greenteeth a.k.a. Wicked Jenny, Ginny Greenteeth and Grinteeth is a figure in English folklore. A river-hag, similar to Peg Powler and derived from the grindylow, she would pull children or the elderly into the water and drown them. The name is also used to describe pondweed or duckweed, which can form a continuous mat over the surface of a small body of water, making it misleading and potentially treacherous, especially to unwary children. With this meaning the name is common around Liverpool and southwest Lancashire.

Jenny Greenteeth was often described as green-skinned, with long hair, and sharp teeth. She is called Jinny Greenteeth in Lancashire and north Staffordshire but in Cheshire and Shropshire she is called Wicked Jenny, Ginny Greenteeth or Jeannie Greenteeth. She is also described as lurking in the upper branches of trees at night, although this may be a folklorist's confusion with the northern English Jinny-hewlet, a folk name for an owl.

Her name is derived from that of the Grindylow which has the alternate name "Grendith" or "Grinteeth" which eventually developed into the term "Jenny Greenteeth".

She is likely to have been an invention to frighten children from dangerous waters, similar in nature to the Slavic Rusalka, the Kappa in Japanese mythology, or Australia's Bunyip. Some folklorists believe the tale recalls sacrificial practices. A similar figure in Jamaican folklore is called the River Mumma (River Mother). She is said to live at the fountainhead of large rivers in Jamaica sitting on top of a rock, combing her long black hair with a gold comb. She usually appears at midday and she disappears if she observes anyone approaching. However, if an intruder sees her first and their eyes meet, terrible things will happen to the intruder. A similar figure known as the Storm Hag (uncommonly also known as Jenny Greenteeth) appears in American folklore around Lake Erie, specifically in the urban legends of Erie, Pennsylvania, in which sailors use a paranormal being to explain the dangers and shipwrecks in the Erie Triangle. The Storm Hag is said to be a green skinned woman with teeth like a shark's but green, as well as piercing yellow eyes, who rests on the bottom of the Lake, off the coast of Presque Isle and sings a song whenever a ship approaches.

Come into the water, love,
Dance beneath the waves,
Where dwell the bones of sailor-lads
Inside my saffron cave.

— S. E. Schlosser, Spooky Pennsylvania

When the sailors of the ship hear the song, the Storm Hag attacks the ship and crew with violent storms and waves, sinking and devouring them.

Jenny Greenteeth inspired the lake monster Meg Mucklebones in the 1985 Ridley Scott fantasy film Legend.