A tin-containing metallic complex derived from Curcuma longa, bis[1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dionato-κO,κO ′ ]bis(butyl), has been obtained and used as a green corrosion inhibitor for carbon steel in 0.5 M sulfuric acid by using weight loss, electrochemical techniques, and the Density Functional Theory. It was found that the obtained metallic complex greatly decreases the steel corrosion rate by adsorption according to a Frumkin model in a weak, physical type of adsorption. Inhibitor efficiency increased with its concentration, and it acted as a mixed type of inhibitor. Results were supported by quantum-chemical research in order to examine the relationship between structural and electronic properties and the inhibitor efficiency.