The electrochemical sulfide oxidation reaction (SOR) offers a promising route for simultaneous sulfide remediation and value-added sulfur production while serving as a thermodynamically favorable alternative to the oxygen evolution reaction (OER), which is a key bottleneck in water electrolysis. However, the practicability of the SOR is often limited by sluggish reaction kinetics, complex reaction intermediates, and catalyst deactivation caused by surface poisoning. Herein, we report the efficient, selective, and stable SOR performance of a new ternary heavy-metal sulfide electrocatalyst, Ba6Ni25S27 (BNS), featuring an interconnected three-dimensional Ni–S network. Cubic Ba6Ni25−δS27+δ nanosheets with Ni-deficient and S-rich stoichiometry were synthesized via a facile polyol-assisted solvothermal method. Electrochemical studies revealed that BNS strongly favors the SOR over the OER, achieving a current density of 10 mA cm−2 at a potential of 0.5 V vs. RHE, which is 1.35 V lower than that required for the OER. BNS exhibits faster kinetics for the SOR (a Tafel slope of 61 mV dec−1) than for the OER (a Tafel slope of 197 mV dec−1), as supported by impedance data and temperature-dependent electrochemical studies. Moreover, potential-dependent in situ Raman spectroscopy, coupled with theoretical calculations, confirms the dynamic evolution of polysulfide intermediates during the SOR and further validates the intrinsic activity of the Ni–S framework for sulfide redox chemistry. The catalyst also exhibits excellent structural and compositional stability during 128 h of continuous operation of the SOR. This work establishes BNS as a new ternary sulfide electrocatalyst with intrinsic selectivity toward the SOR, providing design principles for developing efficient alternatives to the OER in sustainable electrochemical technologies.