The aim of this work was to investigate to what extent the performance of a glass fiberreinforced polymer (GFRP) sandwich slab-bridge with uniform high-density balsa core could be improved in terms of structural efficiency and weight thanks to a more complex core assembly. This core consisted of high-density and low-density balsa and a fiber-reinforced polymer (FRP) arch inserted into the balsa high-/low-density interface. Quasi-static load-bearing experiments on sandwich arch beams with complex core assemblies under symmetric four-point and asymmetric three-point loading were performed. The FRP arch reduced the force in the upper face sheet in the mid-span region and thus prevented compression failure of the latter, which led to a higher ultimate load. It also contributed to the shear resistance by up to 20% for symmetric loading. The best overall performance in terms of structural efficiency (stiffness and resistance) and weight resulted from a core configuration with a FRP arch between an upper high-density and lower low-density balsa core.