The type material of seven taxa of Pleurosigma W. Smith were studied, including P. atlanticum Heiden & Kolbe, P. chilense Hustedt & Krasske, P. chilensis var. patagonica Ferrario & Sar, P. indicum Simonsen, P. intermedium W. Smith, P. nubecula W. Smith and P. simonsenii Hasle. All these taxa are mainly characterized by a straight or almost straight valve outline. The fine morphology of the type specimens is studied and compared with similar taxa, and appropriate taxonomic changes are proposed. The material examined included the lectotypes of P. chilense and P. atlanticum, the holotypes of P. indicum and P. simonsenii from the Friedrich Hustedt Center for Diatom Research (BRM, Germany), the isolectotypes of P. intermedium and P. nubecula from the Van Heurck Collection (BR, Belgium), and the holotype of P. chilensis var. patagonica and material of P. simonsenii collected in Argentinian coastal waters from the Herbarium of the División Ficología (LPC, Argentina). Based on light and scanning electron microscope analyses, it was concluded that P. atlanticum, P. chilense, P. indicum, P. intermedium and P. simonsenii are valid species, P. nubecula is a heterotypic synonym of P. intermedium and P. chilensis var. patagonica must be raised to specific rank under the name P. patagonicum (Ferrario & Sar) Sterrenburg & Sar stat. nov. as it differs from P. chilense in the fine morphology of the external central raphe fissures, the valve apex and the areolae near the centre.