A siphonophore (from Ancient Greek σίφων (siphōn), meaning "tube" and -φόρος (-phóros), meaning "bearing") is a member of the order Siphonophorae. According to the World Register of Marine Species, the order contains 175 species described thus far.

Siphonophores are highly polymorphic and complex organisms. Although they may appear to be individual organisms, each specimen is in fact a colonial organism composed of medusoid and polypoid zooids that are morphologically and functionally specialized. Zooids are multicellular units that develop from a single fertilized egg and combine to create functional colonies able to reproduce, digest, float, maintain body positioning, and use jet propulsion to move. Most colonies are long, thin, transparent floaters living in the pelagic zone.

Like other hydrozoans, some siphonophores emit light to attract and attack prey. While many sea animals produce blue and green bioluminescence, a siphonophore, Erenna sirena, was only the second life form found to produce a red light (the first being the scaleless dragonfish Chirostomias pliopterus).

Siphonophores are colonial hydrozoans that do not exhibit alternation of generations but instead reproduce asexually through a budding process. Zooids are the multicellular units that build the colonies. A single bud called the pro-bud initiates the growth of a colony by undergoing fission. Each zooid is produced to be genetically identical; however, mutations can alter their functions and increase diversity of the zooids within the colony. Siphonophores are unique in that the pro-bud initiates the production of diverse zooids with specific functions. The functions and organizations of the zooids in colonies widely vary among the different species; however, the majority of colonies are bilaterally arranged with dorsal and ventral sides to the stem. The stem is the vertical branch in the center of the colony to which the zooids attach. Zooids typically have special functions, and thus assume specific spatial patterns along the stem.

Siphonophores typically exhibit one of three standard body plans matching the suborders: Cystonectae, Physonectae, and Calycophorae. Cystonects have a long stem with the attached zooids. Each group of zooids has a gastrozooid. The gastrozooid has a tentacle used for capturing and digesting food. The groups also have gonophores, which are specialized for reproduction. They use a pneumatophore, a gas-filled float, on their anterior end and drift at the surface of the water or stay afloat in the deep sea. Physonects have a pneumatophore and nectosome, which harbors the nectophores used for jet propulsion. The nectophores pump water backwards in order to move forward. Calycophorans differ from cystonects and physonects in that they have two nectophores and no pneumatophore. Instead they often possess oil-filled glands which likely help with buoyancy.

Siphonophores possess multiple types of zooids. Scientists have determined two possible evolutionary hypotheses for this observation: 1. As time has gone on, the amount of zooid types has increased. 2. The last common ancestor had many types of zooids and the diversity seen today is due to loss of zooid types. Research shows no evidence supporting the first hypothesis, and has seen some evidence in support of the second.

Currently, the World Register of Marine Species (WoRMS) identifies 175 species of siphonophores. They can differ greatly in terms of size and shape, which largely reflects the environment that they inhabit. Siphonophores are most often pelagic organisms, yet level species are benthic. Smaller, warm-water siphonophores typically live in the epipelagic zone and use their tentacles to capture zooplankton and copepods. Larger siphonophores live in deeper waters, as they are generally longer and more fragile and must avoid strong currents. They mostly feed on larger prey. The majority of siphonophores live in the deep sea and can be found in all of the oceans. Siphonophore species rarely only inhabit one location. Some species, however, can be confined to a specific range of depths or an area of the ocean.

Siphonophores use a method of locomotion similar to jet propulsion. A siphonophore is a complex aggregate colony made up of many nectophores, which are clonal individuals that form by budding and are genetically identical. Depending on where each individual nectophore is positioned within the siphonophore, their function differs. Colonial movement is determined by individual nectophores of all developmental stages. The smaller individuals are concentrated towards the top of the siphonophore, and their function is turning and adjusting the orientation of the colony. Individuals will get larger the older they are. The larger individuals are located at the base of the colony, and their main function is thrust propulsion. These larger individuals are important in attaining the maximum speed of the colony. Every individual is key to the movement of the aggregate colony, and understanding their organization may allow us to make advances in our own multi-jet propulsion vehicles. The colonial organization of siphonophores, particularly in Nanomia bijuga confers evolutionary advantages. A large number of concentrated individuals allows for redundancy. This means that even if some individual nectophores become functionally compromised, their role is bypassed so the colony as a whole is not negatively affected. The velum, a thin band of tissue surrounding the opening of the jet, also plays a role in swimming patterns, shown specifically through research done on the previous mentioned species N. bijuga. The velum becomes smaller and more circular during times of forward propulsion compared to a large velum that is seen during refill periods. Additionally, the position of the velum changes with swimming behaviors; the velum is curved downward in times of jetting, but during refill, the velum is moved back into the nectophore. The siphonophore Namonia bijuga also practices diel vertical migration, as it remains in the deep-sea during the day but rises during the night.