Vapor-phase atomic layer deposition (ALD) of nickel sulfide (NiS x ) is comprehensively reported for the first time. The deposition process employs bis­(N,N′-di-tert-butylacetamidinato)­nickel­(II) and H2S as the reactants and is able to produce fairly smooth, pure, godlevskite-structured NiS x thin films following an ideal layer-by-layer ALD growth fashion for a relatively wide process temperature range from 90–200 °C. Excellent conformal coating is demonstrated for this ALD process, as the deposited NiS x films are able to uniformly and conformally cover deep narrow trenches with aspect ratio as high as 10:1, which highlights the general and broad applicability of this ALD process for fabricating complex 3D-structured nanodevices. Further, we demonstrate the applications of this ALD NiS x for oxygen-evolution reaction (OER) electrocatalysis. The ALD NiS x is found to convert to nickel (oxy)­hydrate after electrochemical aging, and the aged product shows a remarkable electrocatalytic activity and long-term stability, which is among the best electrocatalytic performance reported for nonprecious OER catalysts. Considering that ALD can be easily scaled up and integrated with 3D nanostructures, we believe that this ALD NiS x process will be highly promising for a variety of applications in future energy devices.