The vegetative and male reproductive anatomy of a marine alga, Laurencia intercalaris sp. nov. (Rhodomelaceae, Rhodophyta), is described from subtidal habitats of eastern and southern Korea. This species has terete thalli with entangled fibrous holdfasts and regularly alternate branching of ultimate branchlets, and is inseparable from L. okamurae Yamada on the basis of habit. Vegetative axial cells produce a trichoblast and four pericentral cells in an alternating sequence. Spermatangia are produced intercalary or subterminally from one of two laterals on suprabasal cells of trichoblasts arising from axial cells in apical pits of branchlets. The other lateral remains sterile. In this sterile lateral, budding-like regeneration occurs on older segments that are oabscised. Comparison is made with other related Laurencia species, particularly those with terete thalli. The vegetative anatomy and the regeneration in sterile laterals of male trichoblasts, with the mode of spermatangial formation, distinguish the new species from previously described species of Laurencia including L. okamurae.