In this study, the effect of different honeycomb core geometries on the mechanical behavior of sandwich panels made of PLA and fabricated using 3D printing technology was investigated. For this purpose, three geometric models, including a conventional hexagonal structure, a hexagonal structure with reinforced corners characterized by a fillet radius R, and an octagonal structure, were designed using SolidWorks software and subsequently manufactured. The fabricated specimens were subjected to three point bending and Charpy impact tests to evaluate their performance under flexural and dynamic loading conditions. The results obtained in the present study were validated through comparison with previously published research. The results of the bending and impact tests showed that the reinforced hexagonal model exhibited the highest flexural strength, stiffness, and impact energy absorption owing to its higher structural density. In contrast, the octagonal model, with its more open structure, exhibited the lowest mechanical performance. Furthermore, the impact energy absorbed by the reinforced hexagonal model was 57% higher than that of the octagonal model. These findings highlight the importance of core geometry in optimizing the mechanical properties of lightweight 3D-printed structures.