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Combining inhibitors is a key strategy to increase inhibitory efficacy because of synergistic effects. In this study, a green pharmaceutical inhibitor, amoxicillin (AML), was screened and compounded with potassium iodide (KI) to successfully obtain a synergistic and highly efficient green inhibitor. The results revealed that AML/KI have excellent anticorrosion ability for Q235 steel in 1 M HCl medium. At the optimal concentration ratio of 2 mM AML +5 mM KI, the inhibition efficiency was up to 97.7%, which was significantly higher than that of AML alone (57.4%). The hybrid inhibitor remarkably hinders the corrosion process by inhibiting both the cathodic and anodic reactions. Scanning electron microscopy, electron dispersive spectroscopy, contact angle measurement, ultraviolet–visible spectrometry and X-ray photoelectron spectroscopy, as well as Fourier transform infrared spectroscopy, results further confirm the outstanding inhibitory effect and the formation of synergistic adsorption films. Through density functional theory (DFT) calculations, we discovered that the KI salt significantly enhances the adsorption characteristics of AML/KI on the Fe surface, which is consistent with the experimental findings. This research paves the way for a new breed of green hybrid corrosion inhibitors to be created.