Morphological and anatomical characters of Amphiroa rigida Lamouroux were investigated with field materials and taxonomic accounts are given to the species. The species grows in the subtidal zone of Cheju island, and is characterized by semiendophytic habit in Hydrolithon onkodes. The species is well defined by about 1 cm height, loosely tufted thallus with irregular branches, two-tiered geniculum with equal length, geniculum tier connecting by oblique end walls, and conceptacles buried in cortices of intergeniculum. Korean isolates have etrasporangial, male and female conceptacles. The structure and developmental patterns of conceptacles are very similar to those shown in previous studies.