ABSTRACT.—A new sacoglossan species of the genus Elysia Risso, 1818 from southern Italy is described based on its external morphology, color pattern, radular teeth, and reproductive system. Specimens of this new species were previously attributed to Elysia viridis (Montagu, 1804) because of morphological similarity, but maximum-likelihood and Bayesian analyses based on four genes
(mitochondrial cytochrome c oxidase subunit I, 16S rRNA, nuclear 28S rRNA, Histone 3), including 28 additional species of Elysia from the Atlantic, showed that they are distinct. In addition, to determine the identity of these specimens,
different species delimitation analyses [Automatic Barcode Gap Discovery (ABGD), General Mixed Yule Coalescent (GMYC), Bayesian Poisson Tree Processes, (bPTP)] were carried out. The molecular results support the validity of this new species of Elysia from the Mediterranean Sea, named Elysia rubeni sp. nov. This new species is characterized by a bright light greenish surface color due to the abundance of kleptoplasts. It can be distinguished by its bright blue specks and minute sparkling yellow dots scattered along the body, as well as its smooth and greyish rhinophores.