Chorda rigida Kawai et Arai, sp. nov. (Chordaceae, Laminariales) is described from the Sea of Japan, NW Pacific. This species resembles Chorda filum (Linnaeus) Stackhouse but is distinguished by the following characteristics: 1) the sporophytes grow on more or less exposed rocks at 2–7 m depth and do not form dense tufts; 2) compared with C. filum, sporophytes of C. rigida are much more rigid and are composed of denser cortical layers (6–18 cells thick); 3) C. filum becomes fertile and disappears in late spring to summer, whereas C. rigida appears in early summer, oversummers, and becomes fertile only in late autumn at the same localities; 4) in culture, C. rigida sporophytes tolerate higher temperature conditions (20 and 25° C) than C. filum; and 5) C. rigida has considerably longer sequences of the rDNA ITS region than does C. filum. The independence of the species is further supported by molecular phylogenetic analyses using sequence of the ITS + 5.8S ribosomal DNA. Interestingly, C. filum is shown to be genetically diverse and possibly paraphyletic, and it may require subdivision into several species or subspecies. The rbcL and associated spacer sequence data established monophyly of the genus Chorda among Laminariales, but the resolution was limited for discussing the phylogenetic relationships within the genus.