The chlor-alkali process plays an essential and irreplaceable role in the modern chemical industry due to the wide-ranging applications of chlorine gas. However, the large overpotential and low selectivity of current chlorine evolution reaction (CER) electrocatalysts result in significant energy consumption during chlorine production. Herein, we report a highly active oxygen-coordinated ruthenium single-atom catalyst for the electrosynthesis of chlorine in seawater-like solutions. As a result, the as-prepared single-atom catalyst with Ru-O moiety (Ru-O SAM) exhibits an overpotential of only ~30 mV to achieve a current density of 10 mA cm in an acidic medium (pH = 1) containing 1 M NaCl. Impressively, the flow cell equipped with Ru-O SAM electrode displays excellent stability and Cl selectivity over 1000 h continuous electrocatalysis at a high current density of 1000 mA cm . Operando characterizations and computational analysis reveal that compared with the benchmark RuO electrode, chloride ions preferentially adsorb directly onto the surface of Ru atoms on Ru-O SAM, thereby leading to a reduction in Gibbs free-energy barrier and an improvement in Cl selectivity during CER. This finding not only offers fundamental insights into the mechanisms of electrocatalysis but also provides a promising avenue for the electrochemical synthesis of chlorine from seawater electrocatalysis.