Ni-sulfide compounds synthesized on Ni foam by simple thermal sulfurization are employed as electrocatalysts for water oxidation, resulting in superior activity in alkaline electrolyte media. The role of sulfur in Ni-sulfide was found to be an activator that transformed sulfide into hydroxide, which was eventually transformed into (oxy)hydroxide. The Ni-(oxy)hydroxide phase was also found to be layered and/or amorphous. This activated catalyst showed significant enhancement in water oxidation performance with a low overpotential value of 256 WV at current density of 10 mA cm -2. Our observation could offer important insight into metal-chalcogenide electrocatalyst for water oxidation.