Ni-based materials as highly efficient and low-cost electrocatalysts for hydrogen as well as oxygen evolution reactions (HER & OER) have been developed to produce high-purity hydrogen fuel. Bifunctional catalysts with earth abundant elements often show the quite large applied overpotential originated from slow kinetics on cathode for HER and anode for OER for the overall water splitting reaction in alkaline solutions. Herein, we report that crystalline Ni3S2 thin films have been synthesized using low-temperature atomic layer deposition (ALD) at 250oC without any post annealing. A new ALD chemistry was demonstrated by using bis(1-dimethylamino-2-methyl-2-butoxy) nickel (II) (Ni(dmamb)2) and H2S as precursors. Homogeneous and conformal depositions of Ni3S2 films were achieved on 4-inch wafer. Owing to their high conductivity and stability, crystalline Ni3S2 films were found to be highly efficient bifunctional electrocatalyst for both HER and OER in alkaline solutions. The performance of planar ALD Ni3S2 films is among the best performance for overall water splitting up to date. As-deposited Ni3S2 displays the efficient catalytic activity and stability in HER even on non-conductive substrate, e.g., SiO2. We obtained the low overpotential of 300 mV and high turnover frequency for HER. On the other hand, our ALD Ni3S2 is also capable to serve as high efficient and stable electrocatalyst for OER with overpotential of 400 mV. The full cell two-electrode electrolyzers were constructed using Ni3S2 films as both cathode and anode. And it is able to maintain stable potential of 2.0 V at current density of 10 mA/cm2 for 100 hours.