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We report metallic NiPS₃@NiOOH core−shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS₃ nanosheets are obtained by exfoliation of bulk NiPS₃ in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS₃@ NiOOH core−shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS₃ @NiOOH core−shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS₃ nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS₃ nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS₃@ NiOOH core−shell heterostructure, where the presence of P and S modulates the surface electronic structure of Ni in NiPS₃, thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS₃@ NiOOH core−shell electrode materials for the electrochemical water oxidation.