An adjustable modular shelving system is secureable to a rail mounting system in the cargo area of a vehicle. The shelving system includes a pair of end panels, each pair having an end panel flange with a plurality of end panel flange apertures. The end panel flange extends along a portion of an outer periphery of each of the pair of end panels. The shelving system may include a shelf having a base and a pair of shelf flanges which extend from longitudinal end portions of the base, one of the pair of shelf flanges includes a plurality of locking mechanisms and another one of the pair of shelf flanges include a plurality of shelf apertures. The locking mechanisms may securably connect with a first side of the pair of end panels, and the plurality of shelf apertures may securably connect with a second side of the pair of end panels.