An imidazole-based antifungal drug, namely ketoconazole (KETO), was evaluated as a corrosion inhibitor for steel in 1M hydrochloric acid by electrochemical impedance spectroscopy (EIS) and gravimetric method. Atomic force microscope was also utilized to investigate the corrosion inhibition properties and film formation behavior of KETO. The results showed that KETO is a good corrosion inhibitor for steel in acidic solution at different temperatures. The adsorption followed the Langmuir isotherm, and the thermodynamic parameters were also determined and discussed. Quantum chemical approach was also used to calculate some electronic properties of the molecule in order to ascertain any correlation between the inhibition effect and molecular structure of KETO.