Ketoprofen (KETO) was studied as a corrosion inhibitor for copper in 0.5 M HCl solution using experimental (potentiodynamic polarization curves) and quantum chemical methods. The results obtained from the experiments demonstrated that inhibition efficiency increases with an increase in the concentration of KETO. Additionally, the results of potentiodynamic polarization curves demonstrated that KETO is a mixed-type inhibitor. The adsorption of KETO on a copper surface followed a Langmuir adsorption isotherm. The adsorption equilibrium constant and adsorption free energy were calculated from a Langmuir adsorption isotherm. Density functional theory (DFT) was used for theoretical calculations. The corrosion inhibition efficiencies of non-protonated (neutral) and protonated forms of KETO were determined in both the gas and aqueous phases. The calculated parameters for the various protonated and non-protonated KETO forms were compared with one another.