Cymatonautilus is a genus of cymatoceratids from the middle and upper Jurassic of Europe, the Middle East and Asia characterized by a robust shell with a wide umbilicus and subquadrate whorl section, slightly wider than high. The flanks and venter are flattened. The flanks have a wide lateral groove; the venter a more narrow median groove. The suture has a shallow ventral lobe and broad concave lateral lobes, crossed obliquely by sinuous ribs that produce a deep ventral sinus. The siphuncle is subcentral.