Diagonal fan having a fan housing, within which an external-rotor motor and an impeller are accommodated, wherein the external-rotor motor has a stator and a rotor which at least partly surrounds the stator, and an axial flow duct runs between the fan housing and the external-rotor motor as far as a discharge opening, surrounding the external-rotor motor, of the diagonal fan, through which duct, during operation, air which is drawn in by means of the impeller can be conveyed to the discharge opening, and wherein the impeller is integrated in the rotor.