Velella is a cosmopolitan genus of free-floating hydrozoans that live on the surface of the open ocean.
Velella is commonly known by the names sea raft, by-the-wind sailor, purple sail, little sail, or simply Velella.
Currently the only known and identified species is the Velella velella. These small cnidarians are part of a specialised ocean surface community which includes the better-known cnidarian siphonophore, the Portuguese Man o' War. Specialized predatory gastropod mollusks prey on these cnidarians. Such predators include nudibranchs (sea slugs) in the genus Glaucus and purple snails in the genus Janthina.