All opisthobranchs are hermaphroditic. Many copulate in pairs, exchanging sperm between individuals, while some species form chains of copulating animals. After mating, each individual deposits a gelatinous mass with a variable number of eggs depending on the species. The egg mass resembles a ribbon in which each egg is ecncased in a capsule.

Most opisthobranchs undergo planktotrophic development, with a free-swimming larval stage that feeds on plankton for more than 30 days. Some cases of short lectotrophic development also exist where larvae do not feed, and yet others undergo "direct" development; that is to say, there is no larval stage. The reproductive system is made up of both masculine and feminine organs that mature concurrently.