The invention is an expandable box-shaped spinal implant used to fuse adjacent vertebra in the patient's spine. The implant includes three interlocking parts, which are the superior section, the inferior section, and the U-shaped expander cap. The Box-shaped expandable spinal implant is formed by mating the superior section and the inferior section. The superior section has a top, a pair of mirror image vertical sides, and an upper proximal face. The inferior section has a bottom, a pair of mirror image vertical sides, and a lower proximal face. The pair of mirror image vertical sides on the superior section and the pair of mirror image vertical sides on the inferior section overlap each other and have guide way means and locking means to allow the upper section and the lower section to slideably expand transversely relative to each other. The superior and inferior sections form a closed end and an open end. A pair of exterior longitudinal channels are formed where the pair of vertical sides overlap. The U-shaped expander cap is formed by a pair of opposed prongs and a sealing cap for coupling with the superior and inferior sections. The pair of opposed prongs on the expander is positionable in the pair of expandable channels on the sides of the superior and inferior sections to force the sections slightly apart as the cap abuts the open end of the sections. The U-shaped expander locks the sections together while the cap on the expander seals off the open end of the implant.