A division of gastropod Mollusca, in which the breathing organs are usually situated behind the heart. It includes the tectibranchs and nudibranchs.

gastropods having the gills when present posterior to the heart and having no operculum: includes sea slugs; sea butterflies; sea hares

In zoology, the Opisthobranchia (Milne Edwards, 1848) (also known as opisthobranchs) used to be a subclass of gastropods, within the phylum Mollusca, but they are now treated as an order.