The Portuguese man o’ war (Physalia physalis), also known as the Portuguese man-of-war, Man-Of-War, or bluebottle, though often mistaken as a jellyfish, is a marine cnidarian of the family Physaliidae. The man o’ war is not a jellyfish but a siphonophore, which differs from jellyfish in that it is not actually a single organism, but a colonial organism made up of many minute individuals called zooids.  Each of these zooids is highly specialized, and, although structurally similar to other solitary animals, they are attached to one another and physiologically integrated to the extent that they are incapable of independent survival.

The Portuguese man o’ war is composed of four types of polyp. One of the polyps, a gas-filled bladder called the pneumatophore (commonly known as the sail), enables the organism to float. This sail is bilaterally symmetrical, with the tentacles at one end, and is translucent, tinged blue, purple, pink, or mauve. The Portuguese man o’ war generates carbon monoxide in its gas gland, filling its gas bladder with up to 13% carbon monoxide. The sail is equipped with a siphon. In the event of a surface attack, the sail can be deflated, allowing the man o’ war to briefly submerge.

The other three polyp types are known as dactylozooid (defence), gonozooid (reproduction), and gastrozooid (feeding). These polyps are clustered. The dactylzooids make up the tentacles that are typically 10 metres (33 ft) in length but can be up to 50 metres (160 ft). The long tentacles “fish” continuously through the water, and each tentacle bears stinging, venom-filled nematocysts (coiled, thread-like structures), which sting and kill small sea organisms such as small fish and shrimp. Contractile cells in each tentacle drag the prey into range of the digestive polyps, the gastrozooids, which surround and digest the food by secreting enzymes that break down proteins, carbohydrates and fats, while the gonozooids are responsible for reproduction.

The stinging, venom-filled nematocysts in the tentacles of the Portuguese man o’ war can paralyze small fish and other prey. Detached tentacles and dead specimens (including those that wash up on shore) can sting just as painfully as the live organism in the water and may remain potent for hours or even days after the death of the organism or the detachment of the tentacle.

The Portuguese man o’ war is a carnivore. Using its venomous tentacles, a man o’ war traps and paralyzes its prey. Typically, men o’ war feed upon small aquatic organisms, such as fish and plankton.

Stings usually cause severe pain to humans, leaving whip-like, red welts on the skin that normally last 2 or 3 days after the initial sting, though the pain should subside after about an hour. However, the venom can travel to the lymph nodes and may cause, depending on the amount of venom, a more intense pain.