In biology, a medusa (also known as a hydromedusa) is a form of cnidarian in which the body is shortened on its principal axis and broadened, sometimes greatly, in contrast with the hydroid or polyp. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below. The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella; the mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum). The digestive cavity consists of the stomach and radiating canals which extend toward the margin; these canals may be simple or branching, and vary in number from few to many. The margin of the disk bears sensory organs and tentacles.

In the class Hydrozoa medusae are the sexual individuals of many species, alternating in the life cycle with asexual polyps, but in Scyphozoa (or jellyfishes proper) and Cubozoa the medusa alone is well developed. Except the freshwater jellyfish, these are the only classes in which medusae appear.