The genus Erythroglossum is characterized by Phycodrys-type apical organization, Polyneura-type procarp, and the presence of a midrib. We collected an unidentified Delesseriaceaen species from deep water off the southern coast of the Korean Peninsula. This alga resembles Polyneura japonica in terms of having broadly flattened thalli with a cylindrical stipe, the presence of a midrib and alternative lateral veins. To confirm the taxonomic status of this entity, we compared the morphological features and rbcL sequences among other species of Erythroglossum and P. japonica. As a result, we assigned the new species, Erythroglossum hyacinthinum, to the genus Erythroglossum because of the presence of a midrib. This species is characterized by an elliptical to obovate blade with a short cylindrical stipe, a conspicuous midrib and alternate veins, margins with numerous microscopic dentations, di-trichotomously branching, and bulish-violet iridescence. The phylogeny of rbcL sequences indicates that E. hyacinthinum is definitely a separate entity, but the genera in the tribe Phycodryeae have inconsistent phylogenetic relationships. This is the first study comparing the molecular phylogeny within the genus Erythroglossum.