<p>Developing active and stable electrocatalysts for the chlorine evolution reaction (CER) is critical for chlor-alkali processes but remains challenging. Here we introduce ultrafine high-entropy alloy nanowires (UF-HEANWs) enriched with atomic steps as efficient CER catalysts for direct chlorine electrosynthesis from seawater. A seawater flow electrolyser equipped with UF-HEANW anodes achieves 98.1% CER selectivity at an industrial-scale current density of 10 kA m⁻², maintaining continuous operation for over 5,500 h. Operando studies reveal that atomic steps in lattice-distorted UF-HEANWs create corner-edge electronic heterogeneity, triggering the in situ generation of high-valent Pt–O sites with localized electronic states and unsaturated coordination. These dynamic active structures enhance chloride adsorption and chlorine desorption, leading to improved activity and selectivity during CER. A techno-economic analysis shows that costs are reduced by 32.8% versus the chlor-alkali industry, with 51.3% less electricity used during electrolysis via high-entropy alloy anodes and 83.1% lower feedstock costs from seawater replacement.</p><p></p>