A diagonal fan having an electric motor, a housing and a diagonal impeller generating a diagonal flow which is deflected into an axial flow direction. The diagonal impeller has impeller blades, an air inlet and an air outlet, wherein the housing forms a flow channel for an airflow generated by the diagonal impeller, which has a non-rotationally symmetric axial section and a cylindrical axial section axially directly adjacent, as seen in the flow direction, wherein an air outlet-side radial outer end of the diagonal impeller is arranged in the cylindrical axial section of the flow channel of the housing and an air gap is provided between the radial outer end and the housing, and wherein the non-rotationally symmetric axial section of the flow channel is arranged in a region of the flow channel which is adjacent to the air-inlet side of the air gap in an axial plane with the diagonal impeller.