High corrosion resistance of alloys is essential for their structural applications; however, most alloys suffer from degradation of their corrosion resistance with the increasing acidity of their surround-ings. Nonetheless, we developed a series of medium-entropy alloys (MEAs) in this work, which ex-hibit high strength, superior fracture toughness and ultra-high corrosion resistance, outperforming the variety of corrosion resistant alloys hitherto reported. Most interestingly, our MEAs exhibit an unusual anti-corrosion behavior and their corrosion resistance increases with acidity in Cl- containing solutions. Through extensive thermodynamic calculations, density functional theory (DFT) simulations and experi-ments, we reveal that the unusual anti-corrosion behavior of our MEAs can be attributed to their surface chemical complexity, which facilitates the physio-chemical-absorption of H2O and O 2 and thus the rapid formation of metastable medium entropy passive films that contain the lowest amount of defects, as compared to the passive films on conventional alloys reported in the literature.