Corrosion inhibition of indole-3-acetic acid on mild steel in acidic medium (0.5 M HCl) containing the desired amount of inhibitor has been investigated at different temperatures by using potentiodynamic polarization, electrochemical impedance spectroscopy, and polarization resistance measurements. The experimental results showed that corrosion potential shifted toward a more negative potential region in the presence of indole-3-acetic acid than that of blank solution. According to the obtained results from all measurements, inhibition efficiency was about 77% with 1.7 × 10−3 M inhibitor present, increasing to about 93% at the 1 × 10−2 M inhibitor concentration. Potentiodynamic polarization measurements showed that the current at anodic and cathodic regions obtains a smaller value in the presence of inhibitor at almost all potentials than that of the blank solution. The degree of the surface coverage was determined by using the calculated corrosion current, and it was found that adsorption process of the studied inhibitor on mild steel surface obeys the Langmuir adsorption isotherm. Corrosion of the mild steel increased with a rise in temperature both in the presence and absence of the inhibitor. Activation energy (Ea), Gibbs free energy (ΔGads), enthalpy (ΔHads), and entropy (ΔSads) of corrosion process were calculated by using experimental measurements. Effect of immersion time to corrosion of mild steel was also tested in this study.