A tonneau cover assembly is provided for a bed of a pickup truck having a pair of spaced apart sidewalls. The tonneau cover assembly includes an elongated rail adapted to be fixedly secured to each of the pair of spaced apart sidewalls. Each of the elongated rails includes a bottom member having at least one opening extending therethrough. A panel includes opposing ends slidably coupled to the elongated rails for selectively covering the bed. A retention member is fixedly secured to each of the opposing ends of the panel for slidingly engaging the elongated rails. Each of the retention members is receivable within the opening along one of the elongated rails to retain the panel in place along the elongated rails.