An air duct defines a first air vent and a second air vent at opposite ends thereof, respectively. The air duct includes a duct body and a movable plate. The duct body defines a through opening in a side thereof. The through hole intercommunicates an interior and an exterior of the air duct. The movable plate is mounted on the side of the duct body adjacent to the through opening. By moving the movable plate, the through opening can be changed between an open state and a closed state freely. An electronic device incorporating the air duct is also provided.