An air guiding device for a motor-vehicle includes a main chamber located at the rear of a front wall of the motor vehicle, and defined between the front wall, a rear wall which is spaced apart from the front wall and two side walls. A first opening is provided on the front wall, for the inlet of air into the main chamber, while at least one second opening on said rear wall guides the air into an engine compartment. Two side ducts extend from an inlet to an outlet giving out on a respective outer side wall of the motor-vehicle. To the second opening there is associated a device for controlling the air flow therethrough. The outlets of the two side ducts are arranged and oriented so as to generate a substantially flattened and vertical air curtain adjacent to each front wheel of the motor-vehicle.