Sporolithon ptychoides Heydrich (Sporolithaceae, Sporolithales), the type species of the genus Sporolithon, is newly reported for Thai waters based on specimens collected from the Gulf of Thailand and the Andaman Sea. A detailed morphological and anatomical account is provided, including comparisons with published data of S. ptychoides and its related species. Epithallial cells are flared. Cells of adjacent filaments connect laterally mostly by secondary pit connections. Tetrasporangia are grouped in sori that occur in patches over the thallus surface. Sori are buried in distinct rows in the thallus. Details of male, female and carposporangial conceptacles of S. ptychoides are described for the first time. Gametangial thalli are monoecious with spermatangia and carposporangia born in uniporate conceptacles. Dendroid spermatangial branches occur on the floor, walls and roof of the male conceptacle chamber. Carpogonial branch consists of a hypogenous cell and a carpogonium. Central fusion cell is absent on the floor of the carposporangial conceptacle chamber.