A tonneau cover is arranged to mount on the truck box of a conventional pick up truck for enclosing the box. The cover comprises a pair of panels which are pivotally mounted on respective sides of the box. The panels extend laterally inward such that free ends of the panels engage each other in a closed position. The panels may be pivoted upward and secured in an upward orientation by a pair of rails connected between the free ends of the respective panels in an extended position of the panels. The panels form a pair of side walls in the extended position for supporting large cargo in the box. The rails form a rack in the extended position for supporting additional cargo thereon. A locking device secures the panels in the closed position while a plurality of struts provides additional support to the panels when pivoting the panels between the closed and extended positions.