A comparative study of the corrosion behavior of type 304 stainless steel (304 SS) in a nutrient-rich simulated seawater-based medium in the presence and absence of a marine aerobic Pseudomonas NOMB 2021 bacterium was carried out. Electrochemical studies (including Tafel plots and electrochemical impedance spectroscopy) revealed that 304 SS underwent different corrosion processes under the attack of Pseudomonas bacteria. In the absence of the Pseudomonas bacterium, the passivating film on 304 SS remained relatively stable, and the anodic reaction was under diffusion control. Colonization of the Pseudomonas biofilm on the coupon surface led to the acceleration of corrosion rates and a dramatic decrease in resistance of passivating film due to localized breakdown of the film. Scanning electron microscopy (SEM) results revealed an increase in heterogeneity and coverage of the biofilms of Pseudomonas bacterium with exposure time. Extensive micropitting corrosion underneath the biofilms was also observed, in comparison to minor pitting corrosion observed on the control coupon surface. Energy dispersive X-ray spectroscopy (EDX) analysis showed enrichment of carbon, oxygen, and chlorine and depletion of the metallic elements, iron, chromium, and nickel in the pitted regions, indicating that pitting corrosion was caused by the synergistic effect of aggressive chloride anions, colonized bacterial cells, and extracellular polymeric substances (EPS).