Recent expeditions to the Atlantic sector of the Southern Ocean have yielded valuable collections of shelf and deep water polyplacophorans. These included several specimens of Stenosemus simplicissimus (Thiele, 1906), a species previously known only by its holotype and type locality at the Cape of Good Hope. The new material enabled a thorough morphological redescription of the species by studying valve, perinotum, and radula characters with SEM. The new records from Shag Rocks and the eastern Weddell Sea enlarge the species' biogeographic distribution from the temperate South African region to the polar South Georgia and Weddell Sea regions. Its bathymetric range is extended from 318 m to 285–1064 m. The limited occurrence of deep-water Antarctic polyplacophorans may be caused by benthic predators that limit the expansion of non-herbivorous chitons in the Antarctic deep sea.