Gillyweed is a magical plant in the Harry Potter series created by J.K. Rowling. When consumed, it grants the ability to breathe underwater to the person who eats it. Gillyweed has long, trailing stems with flat, broad leaves and is native to the Mediterranean Sea. Gillyweed is a notable example of magical flora in the wizarding world, and it plays a crucial role in the series during the Triwizard Tournament.