Anopheles (Nyssorhynchus) oswaldoi (Peryassú, 1922) comprises a species complex in South America. To fully characterize other taxa within the Oswaldoi Complex, it is essential to fix the identity of the nominotypical member. Given that the no type was designated in the original description, a lectotype is formally designated from the remaining syntypes in the Museu Nacional do Rio de Janeiro. These and other recently collected specimens from the type locality (Espírito Santo, Brazil) and the State of São Paulo, Brazil were used to redescribe the species using morphological characters of the adult female, male and male genitalia, and the fourth-instar larva and pupa. The larva, pupa, and male genitalia are illustrated. Diagnostic morphological characters of the adult female and male genitalia are provided to distinguish An. oswaldoi s.s. from the morphologically similar An. konderi,An. galvaoi, and An. ininii. DNA sequence data from the second nuclear internal transcribed spacer region (ITS2) are included to fix the molecular identity of An. oswaldoi s.s. Copyright © 2007. Magnolia Press.