An Adjustable Fin Assembly for Watercraft. The modular assembly should be design to be incorporated into a surfboard or other watercraft during fabrication of the craft. Each fin assembly is designed to allow for the watercraft user or fabricator to attach a conventional fin element to it. The fin assembly enables the user to adjust the fin position longitudinally as well as laterally relative to the watercraft. The housings each include a set screw or other component that can be loosened and tightened by the user in order to reposition the fin and then fix it in the new position. The conventional fin attaches to a sliding plate that is housed within a housing, wherein the movement of the sliding plate relative to the housing is prevented by engagement with the set screw.