Adjustable Air deflector is used for installation with a roof ventilator having a pair of vent parts defining a multiplicity of air passages. The air deflector comprises of a base member and its blocking member extends upwardly from the base member at a generally acute angle of 75°. Two air deflectors extend along the entire length of the roof ventilator proximate to the outer edge of each opposing vent part. The base member is disposed between the bottom surface of the corresponding vent part and the upright airflow-blocking member confronts. It is spaced apart a selected distance from the outer edge of the vent part and angles inward slightly toward the vent part. The top edge of the upright air flow blocking member angles slightly outward away from the vent part, and is disposed at approximately the same height as the top panel of the roof ventilator. The base member includes end notches designed and positioned to facilitate installation and retrofitting of the air deflector with a wide variety of existing roof ventilator configurations, and spacing of the upright airflow-blocking member at desired distances from the outer edge of the vent part.