Gillyweed is a magical plant native to the Mediterranean Sea. When it is eaten by a witch or wizard, one grows gills and webbing between the fingers and toes, allowing them to process oxygen from water and navigate underwater more easily. There is some debate among Herbologists as to the duration of the effects of Gillyweed in fresh water versus salt water,  but the effects of Gillyweed in fresh water seem to last about an hour.
Gillyweed is said to resemble a bundle of slimy, grey-green rat tails.
Harry Potter used Gillyweed during the Second Task of the 1994 Triwizard Tournament. Dobby gave it to him, after overhearing a conversation which Barty Crouch Jr, disguised as Alastor Moody, staged with Professor McGonagall.
Connection with Gillywater
Gillyweed can also, presumably, be used as an ingredient in a drink called Gillywater. Professor McGonagall once ordered a glass of it in The Three Broomsticks. Romilda Vane offered Harry a glass of Gillywater spiked with a Love Potion, in an attempt to get Harry to ask her to Professor Slughorn's Christmas Party.
Behind the scenes
- Chocolate Frog Cards reveal that Gillyweed's properties were first discovered by Elladora Ketteridge but credited to Beaumont Marjoribanks about a century later.
- Gillyweed is featured in Magical Water Plants of the Mediterranean and Magical Water Plants of the Highland Lochs, books lent to Neville Longbottom by Barty Crouch Jr, who was posing as Alastor Moody.
- In the film, Neville Longbottom tells Harry about Gillyweed (he learned of it in a book Moody had given him), and Harry is given the actual Gillyweed by Neville and told to put it in his mouth by Crouch/Moody.
- In the book, it is Dobby who tells Harry about it and gives Harry the Gillyweed, after overhearing Moody/Barty's staged conversation with Professor McGonagall about which tactic the Champions might use in the Second Task.
- There seems to be some Gillyweed stored in the Herbology Greenhouses as well as Professor Snape's private potion stores.
- Harry Potter and the Philosopher's Stone (video game) (Mentioned on a Famous Wizard Card)
- Harry Potter and the Chamber of Secrets (video game) (Mentioned on a Famous Wizard Card)
- Harry Potter and the Goblet of Fire (First appearance)
- Harry Potter and the Goblet of Fire (film)
- Harry Potter and the Goblet of Fire (video game)
- Harry Potter and the Order of the Phoenix (video game)
- LEGO Harry Potter: Building the Magical World (Mentioned only)
- LEGO Harry Potter (Mentioned only)
- The Wizarding World of Harry Potter
- LEGO Harry Potter: Years 1-4
- Harry Potter for Kinect (Mentioned only)
- Harry Potter: The Character Vault