Leptochiton sanmatiensis, new species, is described from shallow-waters of Argentina. The species is characterized by having a quincuncial arrangement of tegmental granules on head valve, lateral areas of intermediate valves and postmucronal area of tail valve, and a longitudinal arrangement of these structures on the central areas of intermediate valves; each granule having a macraesthete and four micraesthetes. In addition, the new species has pectinated dorsal scales, with 14–21 ridges; a low number of ctenidia (up to six on each side in the largest specimens); and the second lateral tooth with three denticles in the cusp. The species lives on small gravel and shells.