Recently deschbed new species, Cladosiphon umezakii Ajisaka (Ectocarpales,Phaeophyceae) is reported in Korea based on morphology and plastid rbcL sequences. Cladosiphon umezakii occurred on low intertidal to subtidal zone along the south and southeast coasts in Korea. Thalli are gold to dark brown, ercct, cylindrical, irregularly branched,and very mucoid. Unitocular zoidangium is developed at the base uf assimilatory filaments. Assimilatory filaments are very long. Eight specimens of the pecies collected from Korea clustered with those of C. umezakii in Japan in rbcL tree. The pecies showed a sister relationship with C. okamuranus. The occurrence of this warm water species is thought an example of northward migration of marine algae caused by global warming. Because C. umezakii is of the economica! importance of the genus as foods and medidnes, the study on mass culture of the species is necessary in korea.