Elysia pusilla is a species of small sea slug, a marine gastropod mollusk in the family Plakobranchidae. It is a sacoglossan.

Elysia pusilla feeds on the calcified green alga Halimeda and incorporates functioning chloroplasts into its body, thus it is known as a solar-powered sea slug. It is found in shallow water in tropical regions of the Indo-Pacific wherever its host species grows.

Elysia pusilla are part of Sacoglossa, also known as the solar powered sea slugs and might be the most common Elysia species. Sacoglossans are marine gastropods that ingest the chloroplast of algae. Sacoglossa has two superfamilies under it: Plakobranchidae and Oxynoace. The distinguishing feature between the two superfamilies is having a shell or being able to store chloroplast. Oxynoacea consists of three shelled families, all of whom don't photosynthesize with the algae they consume. However, they might be able to retain the chloroplast and use it as camouflage. Elysia pusilla is under the superfamily Plakobranchidae. The majority of the organisms in this superfamily are shell less, however, many develop a shell before hatching. Sacoglossa are the only metazoans on earth to exhibit kleptoplasty. The only other organisms known to have this ability are single celled protists.

Elysia pusilla and Elysia stylifera are each other's closest living relative. They are often found together on a calcareous algae called Halimeda. The sea slugs use the chemicals to protect themselves and their eggs.

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