Despite specimens` large size and ease of collection in northeast Asian waters, the species diversity of the genus Grateloupia still needs more research in Korea. We investigated plastid rbcL sequences and carried out detailed morphological observation on flattened halymeniacean red alga collected in twelve locations around Korea and Japan. We describe Grateloupia jejuensis sp. nov. based on the distinct clade with high support in our rbcL tree. Grateloupia jejuensis is characterized by solitary or caespitose habit and flattened thalli with discoid holdfast, cartilaginous texture, and blunt or bifid axis. Grateloupia jejuensis was distantly related to G. elata and G. cornea, which have been morphologically confused with the former, and it formed a sister relationship with Prionitis filiformis from California, USA in the rbcL tree.