Despite the recent description of Stenogramma guleopoense in Korea, the issue of S. interruptum in the northwestern Pacific is still a matter of debate. We analyzed 24 rbcL sequences from specimens collected in Korea, including a longer fragment of rbcL (949 base pairs from the type of S. guleopoense, in addition to morphological observations. Phylogenetic analysis of rbcL sequence revealed a new distinct clade that differed from S. guleopoense from Korea and other species of Stenogramma. The morphology and anatomy of the Korean clade supported its distinct position in the genus Stenogramma. On the basis of these results, we describe S. coreanum sp. nov. from Korea. Stenogramma coreanum is distinguished by a combination of its large size (up to 24 cm) of dichotomous to subdichotomous branches divided up to nine times, wider blades with laciniate segments, one to two layers of cortical cells, two to three layers of medullary cells, a gradient of one to two layers of smaller cells between the cortex and medulla, and discontinuous cystocarps on the centre of the frond. Stenogramma coreanum occurs mostly along the south coast of Korea, whereas S. guleopoense occurs along the west coast. The biogeographic implications of the global diversification in Stenogramma are discussed.