A marine alga Caulerpa racemosa was tested for its anticorrosion potential against mild steel in 1 M HCl solution by weight loss and electrochemical methods such as potentiodynamic polarization and electrochemical impedance spectroscopic studies at various temperatures viz., 303, 313, and 323 K. The results showed that inhibition efficiency increased with increase of extract concentration and decreased with increase of temperature. The adsorption of the molecules of the extract on the mild steel surface was in accordance with the Temkin adsorption isotherm. Polarization studies showed that the extract inhibits mild steel corrosion through mixed mode and electrochemical impedance spectroscopy (EIS) results confirm the adsorption of the extract at mild steel/acid interface. The major phytoconstituent caulerpin, a bis-indole alkaloid isolated and characterized using UV, IR and NMR spectroscopic studies, was tested for anticorrosion activity using polarization, impedance, and atomic force microscopy (AFM) studies. The AFM results confirmed the formation of a protective layer by caulerpin over mild steel surface. Thus the anticorrosion potential of C. racemosa can be correlated to the presence of caulerpin.