The effect of marine alga Padina pavonica extract on corrosion inhibition of brass in phosphoric acid was investigated by weight-loss method, potentiodynamic polarization and electrochemical impedance spectroscopic studies. The inhibition efficiency is found to increase with increasing concentration of alga extract and decreases with rise in temperature. The activation energy, thermodynamic parameters (free energy, enthalpy and entropy change) and kinetic parameters (rate constant and half-life) for inhibition process were calculated. These thermodynamic and kinetic parameters indicate a strong interaction between the inhibitor and the brass surface. The inhibition is assumed to occur via adsorption of inhibitor molecules on the brass surface, which obeys Temkin adsorption isotherm. The adsorption of inhibitor on the brass surface is exothermic, physical, and spontaneous, follows first order kinetics. The polarization measurements showed that the inhibitor behaves as a mixed type inhibitor and the higher inhibition surface coverage on the brass was predicted. Inhibition efficiency values were show a good trend follows with weight-loss, potentiodynamic polarization and electrochemical impedance spectroscopy studies. Surface study techniques FT-IR and SEM were carried out to ascertain the inhibitive nature of the algal extract on the brass surface.