The marine red alga Laurencia japonensis sp. nov. (Rhodomelaceae, Ceramiales) is described from the warm temperate region of Japan. It is characterized by the following combination of features: (1) terete, rigidly cartilaginous, upright axes arising from a primary discoid holdfast and stolon-like branches; (2) tristichous branching; (3) penultimate and ultimate branches showing pronounced adaxial bending; (4) adventitious branches formed chiefly in the axils; (5) the production of four periaxial cells per vegetative axial segment; (6) the presence of longitudinally oriented secondary pit connections between contiguous superficial cortical cells; (7) the absence of projecting superficial cortical cells; (8) the absence of a radially elongated cortical layer; (9) the presence of abundant lenticular thickenings in the walls of medullary cells; (10) the presence of a single corps en cerise per superficial cortical cell and trichoblast cell; (11) a parallel arrangement of tetrasporangia; (12) broadly ovoid cystocarps with the lower half immersed within the parent branch tissue; and (13) distally positioned spermatangial nuclei.