Abstract: A collapsible cap for a pickup truck box having separable front, back and intermediate panel members that sealingly connect along joints by releasable fasteners that may be operated by hand without the need of tools. The intermediate panel members are hinged and form side and top panel portions of the cap which are relatively pivotable whereby the top panel portions may be extended in a coplanar relation to said side panel portions to be storable in a flattened arrangement. Extruded connector strips with sealing gaskets extend along edges of the panel members for tight engagement between the pickup truck box and the cap and at joints between the collapsible cap panel members themselves. Manually operable releasable fasteners connect the cap to the box.