The Portuguese man o' war (genus Physalia), also known as the blue bottle, is commonly thought of as a jellyfish but is actually a siphonophore—a colony of four sorts of polyps. A similar group of animals are the chondrophores. The man o' war's float is bilaterally symmetrical with the tentacles at one end, while the velella is radially symmetrical with the sail at an angle. Also the man o' war has a siphon, while the velella does not.

The Portuguese man o' war has an air bladder that allows it to float on the surface of the ocean. It has no means of propulsion and is pushed by the winds and the current. The bladder is actually a single large polyp, called the pneumatophore, which produces its own gas to stay inflated.

Below the jelly dangle long tentacles, sometimes stretching to over ten metres in length. These tentacles are individual polyps called dactylozooids. They stun and kill small sea creatures using poison-filled nematocysts and use muscles to draw the prey in to the gastrozooids, which are yet another different type of polyp that surround and digest it. Gonozooids, the fourth type of polyp, are responsible for reproduction.

Portuguese man o' war are an important source of food to sea turtles, which are immune to the poison. The Portuguese man o' war can be found on the coast of North America and Europe as well as in other areas of the world.

The Portuguese man o' war has tentacles which can be as long as 55 m (33 ft). Their sting is potentially dangerous to humans; these stings have been responsible for several deaths, but usually only cause intense (if not lethal) pain. Detached tentacles and specimens washed up on shore can sting just as painfully as the full creature in the water.