Gillyweed is a magical plant which, when eaten, can temporarily give a human gills and webbed fingers and toes to help them breathe and swim underwater, with the caveat that while under the effects, they are unable to breathe air. The duration of effect can be variable, but it's said to be over an hour. The effects were first discovered by Elladora Ketteridge, who discovered the properties by accident and nearly suffocated on air after eating the plant, but stuck her head into a bucket of water.

Gillyweed was notably used by Harry Potter during the Triwizard Tournament's second task to enable him to breathe underwater in the Great Lake to rescue Ron Weasley.

It's also likely an ingredient in Gillywater, and used in another drink called Gillyweed Tonic.