The Portuguese man o' war, also known as the man-of-war or bluebottle, is a marine hydrozoan found in the Atlantic Ocean and the Indian Ocean. It is considered to be the same species as the Pacific man o' war or bluebottle, which is found mainly in the Pacific Ocean. The Portuguese man o' war is the only species in the genus Physalia, which in turn is the only genus in the family Physaliidae.

Hydrozoa is a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in freshwater habitats. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.