A convertible pickup truck cargo box system is disclosed that allows a pickup truck to easily transform into a flat bed truck, giving a greater range of utility, versatility, options, and cosmetic appearance for both commercial/work applications and recreational/personal applications in just one vehicle. The convertible pickup truck cargo box system may include a convertible cargo box that has a flat bed, a fold up headache rack, removable side panels, and a removable tailgate assembly. The seam between the side panels and the flat bed may be concealed by side trim molding so that their interface is not seen, giving the normal cosmetic appearance of a standard pickup truck. Thus, the pickup truck can be converted easily from a normal looking pickup truck for recreational and personal applications into a flat bed truck for commercial and work applications by removing and storing the side panels and the tailgate assembly.