Elysia viridis, the sap-sucking slug, is a small-to-medium-sized species of green sea slug, a marine opisthobranch gastropod mollusc in the family Plakobranchidae. This sea slug resembles a nudibranch, but it is not closely related to that clade of gastropods. It is instead a sacoglossan.

Elysia viridis is very similar to the sea hare in its soft winged body and colouration, although Elysia sp. is flatter with no oral tentacles. Elysia viridis is known as a 'sap-sucking slug' and feeds only on a single or limited food source (stenophagous).

Elysia viridis are kleptoplasts taking chloroplasts from the algae they graze on and incorporating them into their own digestive glands cells. Once here the chloroplasts can continue to photosynthesize for months enabling the slugs to exploit contributions from both heterotrophy and phototrophy to support their fitness.