Irukandji jellyfish are tiny and extremely venomous jellyfish that inhabit marine waters of Australia and which are able to fire their stingers into their victim, causing symptoms collectively known as Irukandji syndrome. Its size is roughly no larger than a cubic centimetre (1 cm3). There are three known species of Irukandji: Carukia barnesi and the recently discovered Malo maximus and Malo kingi.

The symptoms of Irukandji syndrome were first documented by Hugo Flecker in 1952 and named after the Irukandji people whose country stretches along the coastal strip north of Cairns, Queensland. The first of these jellyfish, Carukia barnesi, was identified in 1964 by Jack Barnes; in order to prove it was the cause of Irukandji syndrome, he captured the tiny jelly and allowed it to sting him while his son and a lifeguard observed the effects.