The medusa is the free swimming stage in the life cycle of cnidarians, such as jellyfish, sea anemones, and hydroids. A medusa is shaped like a bell or umbrella and swims by pulsations of its body. It has a relatively thick mesoglea. The nervous system may be partly concentrated in nerve rings.

Medusa reproduce sexually. In the Hydra and other Hydrozoa they alternate in the life cycle with polyps, from which they are produced by budding. In the Scyphozoa, which all the common jellyfish, the medusa is the dominant form.