Three species of the marine isopod genus Serolis Leach, 1818, are recorded from the continental shelf of southern Brazil, i.e., S. elliptica Sheppard, 1933, S . completa Moreira, 1971, and S. similis sp. n. S. elliptica is for the first time reported from off Brazil. The new species S. similis sp. n. is described and figured. New features distinguishing S. completa from S. elliptica are pointed out. The extension range of S. completa is considerably enlarged, being the Lat. 29ºS its new southernmost limit of occurrence, while the range of S. elliptica is extended further northernwards to Lat. 23ºS.