Seventeen species of ancylids occur in southern South America, seven in Argentina. Anancylus rosanae, n. gen. et sp., is described based on characters of the shell, radula, jaw and reproductive system. This new genus, found in the upper Iguazú River rapids, Iguazú National Park, Misiones Province, Argentina, has the following distinctive features: patelliform shell, usually low, with a circular apex located at the anterior end of the shell; three adductor muscles: large posterior (twice the length of the anterior muscles) and two tear-shaped anterior; adhesive epithelium between the two anterior muscular insertions; no difference between the plates of the jaw.