Solen tehuelchus Hanley, 1842 is currently the only razor-clam reported from Brazilian waters. Although its shell is well known and similar to the other Solenoidea, their anatomical aspects are poorly studied. Recently some living specimens of S. tehuelchus were obtained from a Brazilian beach, allowing the removal of their testes for an ultrastructural analysis. In order to add a new anatomical feature for this species and provide a brief comparison between some related taxa, the shell and sperm ultrastructure were described in detail. The present study showed that characteristics of the spermatozoa of S. tehuelchus are similar to those of other solenids, although certain sperm ultrastructural characteristics such as a short acrosome with an electron-dense base and rounded nucleus are also similar to the species Tagelus plebeius (Lightfoot, 1786) (Solecurtidae). Herein, we have added new anatomic and taxonomic data hitherto unknown to science. The sperm morphology of S. tehuelchus, might in the near future bring new insights into the systematic of Solenidae.