The use of Reishi mushroom or Ganoderma lucidum (G. lucidum) extract as a green corrosion inhibitor for 1018 carbon steel in 0.5 M H₂SO₄ has been evaluated by using weight loss tests. These tests were complemented by electronic microscopy analysis and infrared spectroscopy. Results have shown that G. lucidum extract acts as a good corrosion inhibitor, with its efficiency increasing with increasing the inhibitor concentration and decreasing with an increase in the testing temperature from 25 up to 60 °C. The corrosion inhibition of G. lucidum was found to be due to the presence of functional groups, mainly amines that are physically adsorbed on to the steel surface by following a Langmuir type of adsorption isotherm, which form a protective film on top of the steel.