A report of a new species and new record of (Nudibranchia, Cadlinidae) from South Korea.

Of the four species in the genus present in the northwestern Pacific region, has been the only species recorded from South Korea. For the purpose of investigating in Korean waters, specimens were collected from the Korean East Sea (Sea of Japan) by scuba diving. The radula and morphology of these specimens were examined by stereoscopic and scanning electron microscopy. Based on morphology, three species were identified in Korean waters, including the new species, , (first record in South Korea), and . somewhat resembles but differs in both its morphology as well as the structure of its radula. The background color of is translucent white, tubercles on the dorsum are opaque white and the yellow marginal band is absent. The radular formula of is 57 × 23.1.23 with a rectangular rachidian tooth. In addition, mitochondrial cytochrome c subunit 1 (COI), 16S ribosomal RNA (16S rRNA), and nuclear 28S ribosomal RNA (28S rRNA) gene sequences were generated and used for analysis of Automatic Barcode Gap Discovery (ABGD) and reconstruction of the phylogenetic tree. Morphological distinction and genetic analyses confirm that three species are present in Korean waters of which is a new species.