A diagonal fan includes an electric motor, a housing, and a diagonal impeller received inside the housing and drivable via the motor. The diagonal flow during operation is deflected in an axial flow direction. The diagonal impeller includes impeller blades distributed in the circumferential direction and a slinger ring encloses said blades. The diagonal fan includes an inlet nozzle on the suction side accommodating a main flow for the diagonal fan. The inlet nozzle extends overlapping at least in sections relative to the radial section of the slinger ring forming a nozzle gap therewith. A bypass channel on the housing forms a flow connection from a pressure-side surrounding region (U) of the diagonal fan to an inflow side of the nozzle gap. During operation of the diagonal fan, a swirl-free secondary flow (NS) is guided at the inflow side of the nozzle gap via the bypass channel.