Sea slug is a common name for some marine invertebrates with varying levels of resemblance to terrestrial slugs. Most creatures known as sea slugs are gastropods, i.e. they are sea snails (marine gastropod mollusks) that over evolutionary time have either completely lost their shells, or have seemingly lost their shells due to having a greatly reduced or internal shell. The name "sea slug" is most often applied to nudibranchs, as well as to a paraphyletic set of other marine gastropods without obvious shells.

Sea slugs have an enormous variation in body shape, color, and size. Most are partially translucent. The often bright colors of reef-dwelling species implies that these animals are under constant threat of predators, but the color can serve as a warning to other animals of the sea slug's toxic stinging cells (nematocysts) or offensive taste. Like all gastropods, they have small, razor-sharp teeth, called radulas. Most sea slugs have a pair of rhinophores—sensory tentacles used primarily for the sense of smell—on their head, with a small eye at the base of each rhinophore. Many have feathery structures (cerata) on the back, often in a contrasting color, which act as gills. All species of genuine sea slugs have a selected prey animal on which they depend for food, including certain jellyfish, bryozoans, sea anemones, and plankton as well as other species of sea slugs.

Sea slugs have brains. For example, Aplysia californica has a brain of about 20,000 nerve cells.