Hydrogen peroxide (H2O2) is widely used as a green oxidant for varying applications. Electrosynthesis is an economical and environmentally-friendly strategy to directly produce H2O2. Its practical production is hindered, however, by the lack of highly efficient and selective as well as low-cost electrocatalysts. Transition metal chalcogenide materials have been proved to be good catalysts for the oxygen evolution reaction, but their selectivity towards conversion of O2to H2O2remains unsatisfactory. In this work, 3 atomic-layer defect-rich CoSe2nanosheets have been successfully grown on carbon cloth as a high performance electrocatalyst. The morphology of CoSe2can be tuned by regulating the wettability of the growth substrate. The atomic-layer CoSe2nanosheets showed unprecedented 92% selectivity towards H2O2production in alkaline solutions, and the yields of H2O2in alkaline and acidic solutions were 1227.7 mg L���1h���1and 894 mg L���1h���1, respectively. The high efficiency and selectivity originate from the rich defects in the atomic-layer CoSe2nanosheets, which can accelerate the adsorption of OOH by the exposed Co defect sites.