Roseolithon sabulosum, a coralline algal species within Hapalidiaceae, has been documented from Japan. Phylogenetic analyses based on rbcL and psbA sequences confirmed that the Korean non-geniculate coralline algal specimens from Jeju Island are conspecific with Japanese R. sabulosum, including the holotype. Additionally, both morpho-anatomical and molecular analyses confirmed the identity of our specimens as R. sabulosum. Roseolithon sabulosum is characterized by flared epithallial cells, variable subepithallial initials, and multiporate tetra/bisporangial conceptacles elevated above the thallus surface with 4-7 rosette cells in conspicuous depressions that produce a pitted appearance. Its conceptacle roofs are composed of 3-6 cell layers, and pore canals are lined by 2-4-celled filaments containing rosette cells with disintegrated roofs underlain by elongate to wedge-shaped cells longer than other roof cells. In this study, we report R. sabulosum for the first time in Korea, thereby broadening its known distribution to include both Korea and Japan.