In this work, the microbiologically influenced corrosion (MIC) of Fe(CoCrMnNi) and Fe(CoCrMnNi) medium entropy alloys (MEAs) induced by Pseudomonas aeruginosa (P. aeruginosa) was investigated. Corrosion behaviors during 14 days of immersion in sterile and P. aeruginosa-inoculated culture media are presented. Under sterile conditions, both MEAs exhibited good corrosion resistance against the culture medium solution. In the presence of P. aeruginosa, the pitting corrosion of MEAs was promoted. The results of inductively coupled plasma‒mass spectrometry (ICP‒MS) and potentiodynamic polarization tests showed that the presence of P. aeruginosa promoted the selective dissolution of passive film and accelerated the corrosion of MEAs. The results of X-ray photoelectron spectroscopy (XPS) and Mott–Schottky measurements further demonstrated the degradation effect of P. aeruginosa on the passive film. Compared with Fe(CoCrMnNi), Fe(CoCrMnNi) manifested better resistance to the MIC caused by P. aeruginosa, which may be attributed to more Cr oxides and fewer Fe oxides of the passive film.