The new genus Neoharaldiophyllum J.C. Kang et M.S. Kim belonging to the tribe Myriogrammeae, subfamily Phycodryoideae of the Delesseriaceae, is described. This new genus consists of four species, namely Neoharaldiophyllum udoense (M.S. Kim et J.C. Kang) gen. et comb. nov. from Jeju Island Korea as the type species, Neoharaldiophyllum nottii (R.E. Norris et M.J. Wynne) J.C. Kang et M.S. Kim comb. nov. from the Hood Canal, Mason County, Washington, USA, Neoharaldiophyllum mirabile (Kylin) J.C. Kang et M.S. Kim comb nov. from Canoe Island, San Juan County, Washington, USA, and Neoharaldiophyllum erosum (Harvey) J.C. Kang et M.S. Kim comb. nov. from Garden Island, Western Australia. The morphological traits of the new genus are very similar to the genera Haraldiophyllum and Myriogramme in terms of the vegetative and reproductive structures. There are differences among the three genera in the developmental patterns of the carposporophyte: the primary gonimoblast cells of Neoharaldiophyllum are prostrate on the floor cells of the cavity of the cystocarp and fuse together secondarily, whereas in the two other genera they remain free without a secondary incorporation with the floor cells; the carposporangia of Myriogramme are borne in short chains terminating the gonimoblast filaments, while those in the other two genera are borne as solitary structures. The molecular phylogenies based on rbc L and LSU sequences strongly support the significance of the developmental patterns of the carposporophyte and support the separation of Neoharaldiophyllum from Haraldiophyllum in the tribe Myriogrammeae.