A cover for a C-shaped truck bed extender has an end panel and a pair of opposed side panels joined to first and second panels to form an enclosure for the extender. The enclosure has an opening, and a closure panel is used to open and close the opening to provide a cover encasing the extender. Pocket structures may be provided to receive ends of the extender's leg sections. The cover may include openings in the side panels to allow passage of trunnions on mounting brackets of the extender.