Velella is a monospecific genus of hydrozoa in the Porpitidae family. Its only known species is Velella velella, a cosmopolitan free-floating hydrozoan that lives on the surface of the open ocean. It is commonly known by the names sea raft, by-the-wind sailor, purple sail, little sail, or simply Velella.

This small cnidarian is part of a specialised ocean surface community that includes the better-known cnidarian siphonophore, the Portuguese man o' war. Specialized predatory gastropod molluscs prey on these cnidarians. Such predators include nudibranchs (sea slugs) in the genus Glaucus and purple snails in the genus Janthina.

Each apparent individual is a hydroid colony, and most are less than about 7 cm long. They are usually deep blue in colour, but their most obvious feature is a small stiff sail that catches the wind and propels them over the surface of the sea. Under certain wind conditions, they may be stranded by the thousand on beaches.

Like other Cnidaria, Velella velella are carnivorous. They catch their prey, generally plankton, by means of tentacles that hang down in the water and bear cnidocysts (also called nematocysts). The toxins in their nematocysts are effective against their prey. While cnidarians all possess nematocysts, in some species the nematocysts and toxins therein are more powerful than other species. V. velella's nematocysts are relatively benign to humans, although each person may respond differently to contact with the nematocyst toxin. It is wise to avoid touching one's face or eyes after handling V. velella, and itching may develop on parts of the skin that have been exposed to V. velella nematocysts.