Aerodynamic device for a motor-vehicle including a panel structure located under the front part of the motor-vehicle and elastically biased towards a raised position and configured to be moved towards a lowered position due to an airflow which invests the motor-vehicle during travel. The aerodynamic device includes a bag structure provided inside of a cavity for receiving in its interior the airflow which invests the motor-vehicle during travel, in such a way that above a predetermined threshold value of the motor-vehicle speed, the bag structure is inflated and pushes the panel structure towards its lowered position.