Sea hares (also called sea slugs) are small marine gastropod molluscs of the suborder Anaspidea (P. Fisher, 1883) in the subclass Orthogastropoda, class Gastropoda, phylum Mollusca. Their common name derives from the projections on their bodies which somewhat resemble rabbit ears. Their scientific name (Anaspidea) is derived from the Greek for "without a shield" and refers to the lack of the head shield which characterises other members of the Opisthobranchia.

Sea hares are large, bulky creatures, reaching a length of up to 75 cm and weight of up to 2 kg. They have soft bodies with an internal shell and are all hermaphroditic, acting as male and female simultaneously, even during mating.

Sea hares are herbivorous and are typically found on seaweed in shallow water. When there is a shortage of food, they burrow themselves. They have an extremely good sense of smell. They follow even the faintest scent with the chemoreceptors on their tentacles.

They rely for protection on their coloration, which camouflages them from predators. When disturbed, a sea hare can release ink from its ink glands, providing a potent deterrent to predators such as sea anemones. This release acts as a smoke screen, while, at the same time, affecting the scent of their predators. The color of the ink is white or reddish, depending on the color of the pigments of the seaweed. Their skin contains a similar toxin that renders sea hares largely inedible for many predators.

Many anaspideans have only a thin, internal and much-reduced shell with a small mantle cavity; some have no shell at all. All species have a radula and gizzard plates.

Aplysia californica is a typical example of a sea hare, and is noteworthy for its usefulness in studies of neurobiology.