Stygiomedusa gigantea is a species of giant deep sea jellyfish in the Ulmaridae family.

With only 114 sightings in the last 110 years it is a jellyfish that is rarely seen, but believed to be widespread throughout the world. It is thought to be one of the largest invertebrate predators in the deep sea ecosystem. The jellyfish has an umbrella-shaped bell that can be up to a metre (three feet) wide, and has four arms that extend up to six metres (20 feet) in length. These “paddle-like” arms lack stinging tentacles, and scientists are unsure of their exact function. It has been suggested that they might be used to envelop and trap prey.