The South-Western portion of Madagascar appears to have one of the highest levels of scorpion diversity on the island. In this paper is presented an analysis of the known species of Grosphus Simon, 1880 from this region. Information on ecological aspects of the species is also given. A new species is also described from the region of Cap Sainte Marie, an area where micro-endemic patterns can be observed. The new species is characterized by a medium size with a total length of 50.3 mm. General coloration yellow to pale yellow without dark zones on body and appendages. Carapace yellow with an anterior reddish-orange zone, approximately forming an inverted triangle. Anterior margin of carapace strongly granular. Male pectines with 36–36 teeth. Fixed and movable fingers of pedipalps with 12–13 oblique rows of granules.