Surface modification of electrocatalysts is a convenient strategy for addressing the drawbacks arising from the sluggish kinetics of the oxygen evolution reaction (OER). However, the preparation of electrocatalysts usually involves complex procedures, rendering investigation of the catalytic mechanism difficult. In this study, S-modified FeOOH electrocatalysts were prepared via a facile hydrothermal method at 120 °C. The electrocatalyst exhibited an outstanding OER performance with an overpotential of 254 mV at 50 mA·cm⁻². Morphological, structural, and electrochemical characterizations revealed that Fe ion is the most important active center. The effect of S on the catalytic process was investigated based on the steric hindrance for OH* adsorption, changes in ΔG of the rate-determining step, and coordination mode of the active species. This is the first report on the formation of an (Fe-S)-O* bond that plays a key role in enhancing the electrochemical performance.