Elysia catulus is a small species of sea slug, a marine gastropod mollusc in the family Plakobranchidae.

This sea slug resembles a nudibranch, but it is not closely related to that order of gastropods, instead it is a sacoglossan.

There are three white streaks on the head, one central and the other two running diagonally past the eye.

There are considerable fluctuations in population size and a sudden increase may be due to the arrival of larvae that have originated elsewhere.

Feeding is done by puncturing the plant's tissue with the radular tooth and sucking out the cell sap.