Novel earth abundant metal sulfate-containing high entropy sulfides, FeNiCoCrXS₂ (where X = Mn, Cu, Zn, or Al), have been synthesized via a two-step solvothermal method. We show that sulfate-containing FeNiCoCrMnS₂ exhibits superior OER activity with exceptionally low overpotential of 199, 246, 285, and 308 mV at current densities of 10, 100, 500 and 1000 mA cm^-2, respectively, and surpassing its unary-, binary-, ternary-, and quaternary-metal counterparts. The electrocatalyst yields exceptional stability after 12000 cycles and 55 h of durability even at a high current density of 500 mA cm^-2. Various in-situ and ex-situ analyses were used to investigate the self-reconstruction of the sulfides during the oxygen evolution reaction (OER) for the first time. The resulting metal (oxy)hydroxide is believed to be the true active center for OER. The remaining sulfate also contributes to the catalytic activity. Density function theory calculation is in a good agreement with the experimental result. The extraordinary OER performance of the high entropy sulfide brings a great opportunity for desirable catalyst design for practical applications.