This paper describes a modular assembly system of GFRP composite structures for potential applications in bridge superstructures. Built-up modular sections consisting of standard GFRP pultruded box or I profiles are incorporated between two GFRP flat panels to form a sandwich structure, which can then be connected in the transverse direction to supporting steel girders to form GFRP-steel composite structures via adhesive bonding and/or mechanical bolting. The modular sandwich system introduced here allows for two pultrusion directions within the one sandwich assembly, therefore an improved structural performance in both the longitudinal and transverse directions is expected. GFRP sandwich specimens with and without foam core were fabricated in this way, using either adhesive bonding or mechanical bolts, and were subsequently tested under four-point bending. The load-carrying capacity, stiffness and effective width of the proposed system were evaluated, and the connection detail and degree of composite action were investigated. Finite element analysis was also performed on the sandwich specimens and showed good comparison with experimental results.