The Atlantic Portuguese man o' war (Physalia physalis), also known as the man-of-war[1], is a marine hydrozoan of the family Physaliidae found in the Atlantic, Indian and Pacific Oceans. Its long tentacles deliver a painful sting, which is venomous and powerful enough to kill fish or (rarely) humans.[2] Despite its appearance, the Portuguese man o' war is not a true jellyfish but a siphonophore, which is not actually a single multicellular organism (true jellyfish are single organisms), but a colonial organism made up of specialized individual animals (of the same species) called zooids or polyps.[3] These polyps are attached to one another and physiologically integrated, to the extent that they cannot survive independently, creating a symbiotic relationship, requiring each polyp to work together and function like an individual animal.

The Indo-Pacific Portuguese man-of-war (P. utriculus), or blue bottle, is a related species with very similar appearance found throughout the Indian and Pacific Oceans.[1]