A cap for the cargo bed of a pickup truck assembled on the truck including a base support frame comprised of a pair of parallel side by side slotted side guide rails and a hoop support frame pivotally mounted to each of the opposite ends of the side guide rails so as to able to be swung up to an erect position after the base support frame is installed on the pickup truck cargo bed side walls. An elongated slotted top guide rail is connected at either end to the top of each hoop support frame. Flexible side panels are inserted in respective slots on the side guide rails and top guide rail assuming a convex shape to cover the cargo bed. End panels close off either end of the cap, a rear panel may be hinged to the rear hoop support frame to be operable and may be transparent to provide a window. The hoop support frames and end panels may be adjustable in width to be matched to the cargo bed width. The cap can be collapsed into a tonneau cover by lowering the hoop support frames and the flexible side panels overly overlapping each other.