Largest box jelly
Flecker's sea wasp Chironex fleckeri
3 metre(s)
Australia ()

The largest species of box jelly is Flecker's sea wasp Chironex fleckeri, with a cuboidal bell that can measure up to 30 cm in diameter, and 60 tentacles that can each measure up to 3 m long in fully extended form when hunting. Native to the waters off Australia, particularly Queensland, it is infamous for being not only the world's most venomous jellyfish but also one of the most venomous of all creatures.

Its venom is so potent that this deadly species has been responsible for at least 63 human deaths in Australia since 1884, yet it was not recognized as a species described by science until 1955.

If stung severely, a person will suffer excruciating pain and can die within 2–5 minutes unless treated straight away.

Box jellies used to be classed within the same taxonomic class, Scyphozoa, as true jellyfishes, but more recently they have been split into their own separate-but-related taxonomic class, Cubozoa.