Microbiologically influenced corrosion (MIC) and chloride ion attack represent principal factors contributing to the premature failure of marine metallic infrastructures. This study systematically investigated the MIC behavior of X80 steel induced by sulfate-reducing bacteria (SRB) in simulated marine environments with varying chloride concentrations. Under sterile anaerobic conditions, elevated chloride concentrations slightly accelerated the corrosion of X80 steel. However, the biotic system (30 g/L Cl−) exhibited an order-of-magnitude increase in the corrosion rate of X80 steel compared to sterile controls. The relative corrosion severity in SRB-inoculated environments followed the descending order: 30 g/L > 10 g/L > 20 g/L > 5 g/L Cl−. Chloride concentration critically modulated SRB metabolic activity. Sulfate reduction rates at 5 g/L Cl− demonstrated a significant 85% reduction relative to optimal conditions at 20 g/L Cl−, correlating with near-complete bacterial growth inhibition. Intriguingly, suboptimal chloride environments (10 and 30 g/L Cl−) triggered substantial extracellular polymeric substance (EPS) production, serving as a protective barrier and ionic exchange medium. EPS exacerbated steel corrosion by accelerating anodic dissolution through complexation with ferrous ions. Fourier-transform infrared spectroscopy results confirmed that EPS contains redox-active functional groups. Injecting 1 g/L EPS into SRB broth increased the corrosion current density from 9.2 ± 0.8 μA/cm2 to 14.3 ± 1.2 μA/cm2. These findings provide new insights into the dual role of EPS in MIC processes, emphasizing its critical function in extracellular electron transfer-MIC mechanisms.