A new species of diatom, Orthoseira limnopolarensis, is described from a sediment core in Limnopolar Lake on Livingston Island, South Shetlands in the South Atlantic Ocean. The new species is distinctive in having an array of numerous (12-14) carinoportulae, a number of clearly defined rimoportulae which are described here for the first time in the genus, and a valve face surface that is covered with small, irregularly scattered spines. Linking spines are less well-developed than is generally observed in species of the genus Orthoseira. No caverns or internal undulations are present. Internal valves have not been encountered.