Sea anemones are a group of predatory marine invertebrates constituting the order Actiniaria. Because of their colourful appearance, they are named after the Anemone, a terrestrial flowering plant. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Hexacorallia. As cnidarians, sea anemones are related to corals, jellyfish, tube-dwelling anemones, and Hydra. Unlike jellyfish, sea anemones do not have a medusa stage in their life cycle.

Cnidaria is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in fresh water and marine environments, including jellyfish, hydroids, sea anemones, corals and some of the smallest marine parasites. Their distinguishing features are a decentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella used mainly for envenomation and capturing prey. Their bodies consist of mesoglea, a non-living, jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. Cnidarians are also some of the only animals that can reproduce both sexually and asexually.