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.