The performance of 5-[(2-chlorophenyl)(hydroxy)methyl]-6-methylpyridazin-3(2H)-one (P4) as corrosion inhibitor for copper corrosion in 2.0 M HNO3 solutions in the temperature range from 303K to 343K was investigated weight loss, electrochemical impedance spectroscopy (EIS) technique and quantum chemical study. The results obtained from the both measurement techniques revealed good inhibitor efficiency in the studied concentration range. The inhibition efficiency decrease with rise in temperature. Impedance measurements showed that the double-layer capacitance decreased and charge-transfer resistance increased with increase in the inhibitor concentration and hence increasing in inhibition efficiency. The free energy of adsorption and the influence of temperature on the adsorption of inhibitor onto copper surface have been reported. The adsorption of the P4 was found to obey the Langmuir adsorption isotherm. Further, theoretical calculations were carried out and relations between computed parameters and experimental inhibition efficiency were discussed.