Abstract: The molten pool structure is a critical feature of laser additive manufacturing and a key factor contributing to the differences in microstructures and properties between the vertical processing direction (XOY plane) and parallel processing direction (XOZ plane) of selective laser melted (SLM) 316L stainless steel. This study investigates the mechanism by which molten pool structures influence the corrosion behavior of SLM 316L stainless steel. Material characterization techniques, including SEM, EBSD, and TEM, were employed to analyze the differences in molten pool structures on the XOY and XOZ planes at different scales. Electrochemical tests and immersion experiments were conducted to evaluate the variations in passivation behavior and pitting resistance between the two planes. The results reveal that the molten pool structure on the XOY plane exhibits a continuous distribution along the processing path, with equiaxed/near-equiaxed cellular substructures inside the molten pools. In contrast, the XOZ plane displays a continuous fish-scale morphology of molten pools, with cellular substructures showing significant orientation dependence. After potentiostatic passivation in 0.5 M H?SO? solution, the passive film on the XOY plane exhibits a lower oxygen vacancy defect density, higher electrochemical impedance values, and a higher relative content of Cr?O?, indicating superior passivation behavior compared to the XOZ plane. Following 12 h of immersion in acidic FeCl? solution, the maximum pit depth on the XOZ plane reaches 447.4 μm, approximately twice that of the XOY plane. Additionally, the edge regions of circular pits on the XOZ plane demonstrate a tendency for pit propagation along the fan-shaped molten pool boundaries. These findings collectively suggest that the fine cellular substructures on the XOY plane promote passive film growth, while the fan-shaped molten pool boundaries on the XOZ plane accelerate pit propagation.