Splitboard boot bindings for backcountry splitboarding. Each of a pair of soft-boot bindings is provided with an integral boot binding lower that conjoins the two halves of a splitboard without the additional weight or height of an adaptor mounting plate, upper binding baseplate or “tray”, and extra fasteners of the prior art. The boot binding lower is formed as a box girder and provides improved torsional stiffness for splitboard riding. When subjected to a torque applied by the rider, the bottom mediolateral edges of the box girders are configured to contactingly engage the top face of the splitboard, thereby dynamically coupling the rider's boot sole and the board via a single rigid structure. In a preferred embodiment, the web or “spacer” members of the box girder are characterized by an aspect ratio or contour height that is varied from heel to toe.