The corrosion inhibition of stainless steel in 5.5 M H3PO4 by clay has been studied as a possible source of ecofriendly inhibitor using potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) methods at 298 K. Results obtained showed that this clay functioned as a corrosion inhibitor for stainless steel in the acidic environment. Inhibition efficiency increased with clay concentration. Polarization curves indicate that this inhibitor acts as a cathodic inhibitor. EIS spectra exhibit one capacitive loop. The charge transfer resistance (Rct) increases with inhibitor concentration, while double layer capacitance (Cdl) decreases. Adsorption of this inhibitor on stainless steel surface obeys the Langmuir adsorption isotherm.