<p>Electrochemical C–S bond construction represents an attractive route to produce valuable organosulfur compounds, yet current approaches suffer from low reaction efficiency and rely on sulfites (SO₃²⁻) as the sulfur source. Here we report a paired electrolysis for hydroxymethanesulfonate synthesis from widespread industrial-waste sulfides (HS⁻). Specifically, HS⁻ is oxidized to SO₃²⁻ by in-situ generated H₂O₂ at cathode, and methanol is oxidized to formaldehyde at anode, with following C–S coupling to form hydroxymethanesulfonate. By matching HS⁻ concentration with current density and strengthening convective mass transfer between intermediates, we achieve efficient hydroxymethanesulfonate production with Faradaic efficiency of 102.8% and productivity of 282 μmol h⁻¹ in a membrane-free H-cell under optimized conditions (50 mA cm⁻², 0.05 M HS⁻, stirring in anodic chamber). By constructing a flow reactor, this paired electrolysis enables conversion of HS⁻-contaminated wastewater (from &lt;10 ppm to 3000 ppm) with Faradaic efficiencies exceeding 100%, while tolerating coexisting anions and heavy metals in real effluents. It also converts waste H₂S gas with &gt;87% conversion and &gt;74% hydroxymethanesulfonate selectivity over 102 hours of stable operation, demonstrating more cost-effective economics to both industrial and previous electrochemical methods.</p>