Abstract: The melt pool structure represents a critical feature in laser additive manufacturing and serves as a primary factor contributing to the disparities in microstructure and performance between the vertical (XOY) and horizontal (XOZ) processing directions of 316L stainless steels fabricated via selective laser melting (SLM). The mechanism of molten pool structure on the corrosion behavior of 316L stainless steels fabricated by SLM was investigated in this study. The differences in molten pool structure on the XOY and XOZ planes were explored by scanning electron microscope (SEM), electron back scattering diffraction, (EBSD), and transmission electron microscope (TEM) in various scales. The electrochemical testing and immersion experiments were conducted to study the differences in passivation and pitting behavior between the XOY and XOZ planes. The results show that the molten pool structure on the XOY plane is continuously distributed along the processing path, with equiaxed or near-equiaxed cellular substructures within the molten pool. In contrast, the molten pool structure on the XOZ plane exhibits a continuous fish-scale pattern, with the cellular structure displaying clear orientation. After forming a film at a constant potential in 0.5 mol/L H₂SO₄ solution, the XOY plane exhibits a passivation film with lower defect density of oxygen vacancy, higher electrochemical impedance, and higher relative content of Cr₂O₃, compared to the XOZ plane, indicating the superior passivation behavior on the XOY plane. After immersion for 12 h in acidic FeCl₃ solution, the maximum pitting depth on the XOZ plane is 447.4 μm, approximately twice than that of the XOY plane. Additionally, the edges of the circular pitting pits on the XOZ plane show the tendency to extend into a fan-shaped molten pool boundary. It can be comprehensively known that the smaller cellular substructure on the XOY plane facilitates the passivation film growth, while the fan-shaped molten pool boundary on the XOZ plane accelerates the pitting expansion.