Abstract: A new Truck Box Accessory Attachment System for offering an easier manner of mounting a truck box accessory to a truck box. The inventive device includes a truck female rail, an attachment male rail, and a slide locking device. In use, the truck female rail (20) is aligned with an alignment tool (70) and attached to a pickup truck (12). The alignment tool (70) is comprised of the support stem (33), the right rail (31), and the left rail (32) and a horizontal tool bar (72) which is fastened at its ends to the support stem (33) by an end attachment fastener (76) and is lengthwise adjustable by virtue of a rail spacing adjustment structure (74). Parallel alignment of the truck female rail (20) at a variety of spaced distances is accomplished by use of the alignment tool (70).