An adjustable side guard assembly that fits into the void space formed on a truck that prevents other motor vehicles or objects from entering the void space. The assembly includes an elongated side plate attached to a rigid frame. The rigid frame has two support members that extend pivotally attached to two laterally extending horizontal members. The distal ends of the two horizontal members engage slots formed on the side plate enabling the side plate to be longitudinally adjusted inside the void space. The opposite ends of the two horizontal members are attached to two upper mounting brackets that are attached to the vehicle frame. A diagonal member may be attached at one end of the horizontal member and at an opposite end to a lower mounting bracket attached to the vehicle frame.