An engine room undercover assembly for a vehicle includes an aerofin mounted on a lower side surface of the undercover to guide airflow under an undercover. In the engine room undercover assembly, the aerofin is configured to maintain a state of being inserted and folded into the undercover when the vehicle is stopped or travels at a speed of less than or equal to a predetermined speed. When the vehicle travels at the speed greater than the predetermined speed, the aerofin protruded to the outside of the undercover to guide the airflow under the undercover.