Sandwich composite are widlely used in various industries such as aerospace and construction industry due to their desirable mechanical properties such as high fatigue and flexural strength and low weight. Nowadays, fiber-reinforced composites play a major role in manufacturing the face sheets of sandwich structures. Among the various methods proposed for the production of these structures, 3D printers are considered as a new solution. The most common type of the 3D printers are fused deposition modelling (FDM) printers, in which the thermoplastic polymer filament is extruded after feeding into a hot block and is deposited layer by layer with a certain geometry. The structures produced by this method do not possess outstanding mechanical properties. Reinforcing the 3D printed structures using nanofibers, short fibers and continuous fibers or yarns is an effective method to enhance their mechanical charactersitics. This study aims to investigate the flexural behavior of fiber-reinforced sandwich structures produced by 3D printing method. In this regard, after designing and manufacturing a continuous fiber 3D printer, 3D sandwich structures with acrylonitrile butadiene styrene (ABS) as matrix and nylon 66 filament as reinforcement were manufactured. The upper and lower face sheets were reinforced with continuous fibers. The core was manufactured in two different geometries i.e triangular and rhombic. In addition, to investigate the effect of foam injection on the flexural behavior of these structures, polyurethane foam was injected in spaces between the core cellular structures. The specimens were then subjected to three-point bending loading. The finding revealed that reinforcing the face sheets with continuous fibers improved the bearing capacity by about 1.5 times and the energy absorption by 6.5 times. Injection of foam into the structure also improved the bearing capacity and energy absorption by more than 50% compared to the reinforced structure without foam. Also, the core geometry did not have a significant effect on the flexural properties of the produced 3D structures. Results also showed that the failure mode in polymeric 3D structures as well as fiber-reinforced structures was the core failure and collapse due to applied shear forces and core-face delamination.