Cubozoa or box jellies are one of the four groups of Cnidaria. They are named after their cube shape with four flattened sides. Most of the about 20 species are found in tropical oceans and seas and are fast, strong and agile swimmers. This swimming ability is due to the velarium, a flap under the umbrella which concentrates and increases the flow of water pushed out from the umbrella. They have four complex eyes that allow them to track moving objects and quickly respond to changes in light intensity.

A flap of tissue called the velarium is located along the underside of the bell. Muscular fleshy pads called pedalia are located at the corners of the bell. One or more tentacles are connected to each pedalium. On the bell, located midway between the pedalia, are four sensory structures called rhopalia.

Box jellies eat zooplankton, fish, worms, and crustaceans. When the tentacles, which can stretch to ten or more times the height of the bell, come into contact with prey, nematocysts, concentrated in rings on the tentacles, fire into the prey's skin. Tiny barbs of the nematocysts hold onto and transfer venom to the prey. As the prey is immobilized by the venom, the tentacle contracts and pulls the prey near to the bell.

Cubozoans are eaten by large fish and sea turtles.

Cubozoans can reproduce sexually. The male puts his tentacles into the bell of the female and passes over packets of sperm. Fertilization takes place inside the females. In some species the fertilized eggs are released into the water column where they develop into larvae (planula), while in other species development into larvae occurs inside the female Box jelly polyps can also reproduce asexually by budding to form new polyps.

Among the box jellies is the Australian sea wasp of (Chironex fleckeri), its sting has caused the deaths of over 60 people since the first report in 1883.