<p>Advanced oxidation processes based on sulfate radical have emerged as an attractive technology in organic dye wastewater treatment. In this work, Co₃O₄ nanosheets with abundant oxygen vacancies and Co²⁺ (r-Co₃O₄) were prepared by a simple precipitation method, followed by a calcination treatment under a hydrogen atmosphere. r-Co₃O₄ shows excellent performance in activating PMS for methylene blue (MB) degradation; 98.8% of MB was degraded in the system at an initial pH 7.00, 75&#xa0;mg L⁻¹ catalyst with 100&#xa0;mg L⁻¹ peroxymonosulfate, and 15&#xa0;mg L⁻¹&#xa0;MB for 30&#xa0;min, yielding a high kinetic constant of 1.172&#xa0;min⁻¹, which was 2.4-fold higher than that of the Co₃O₄ (0.489&#xa0;min⁻¹) calcined under air atmosphere (p-Co₃O₄). More significantly, r-Co₃O₄ shows excellent stability for MB degradation. After four consecutive cycles, the degradation of MB remained above 96.8% under the same reaction conditions. The high specific surface area, large pore size, high Co²⁺ content, and abundant oxygen vacancies might account for the superior performance of r-Co₃O₄ in provoking PMS activation and the subsequent MB degradation. ¹O₂ and SO₄⁻· were identified as the main radical species responsible for MB degradation. This work presents an efficient Co₃O₄ catalyst and a new insight for design of Co₃O₄-based heterogeneous catalysts as activators to activate PMS for dye degradation.</p>