Cyproconazole (CPA) is an azole fungicide and easily biodegradable so that it is an alternative corrosion inhibitor in the water treatment and industrial pickling process. Effects of CPA on the corrosion resistance of copper in 0.5 M H2SO4 solutions were evaluated via electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, and scanning electron microscopy (SEM). Both thermodynamic and adsorption models were calculated and discussed. The results showed that the adsorption of CPA on the copper surface obeys the Dhar–Flory–Huggins and Bockris–Swinkels substitutional adsorption isotherms. The number of water molecules (X) replaced by a molecule of organic adsorbate was determined from the different substitutional adsorption isotherms applied to the data obtained from EIS experiments.