The possibility of using rosemary oil as a corrosion inhibitor of carbon steel in 0.5 M sulfuric acid solution was investigated by determination of weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy. It was shown that the inhibition effectiveness increases with an increase in the concentration of oil and a decrease in the temperature. The inhibiting effect of the investigated oil is due to its adsorption on the surface of the steel and complexation. Adsorption is described by a Langmuir isotherm. The effect of the temperature on the corrosion rate was studied in the presence and absence of the oil and the thermodynamic parameters of the corrosion process were calculated.