The development of photocatalysts enabling stable and highly efficient water splitting hydrogen production remains an open challenge in the field of energy photocatalysis. Herein, Ni2P/γ-Ga2O3 nanosheets have been reported as excellent water splitting photocatalysts. Ni2P particles and γ-Ga2O3 nanosheets were synthesized by a facile hydrothermal process. The Ni2P/γ-Ga2O3 samples were prepared by an electrostatic self-assembly method with Ni2P particles and γ-Ga2O3 nanosheets as precursors. The 0.5 wt% Ni2P/γ-Ga2O3 sample shows a photocatalytic H2-production activity as high as 2.7 mmol g-1 h-1 in pure water and 12 mmol g-1 h-1 in an aqueous methanol solution under a 125 W high pressure mercury lamp, respectively, which are much higher than those of pure γ-Ga2O3 and Pt/γ-Ga2O3 nanosheets modified with a comparable Pt content. The Ni2P component plays a role as an electron collector that promotes efficient separation of photogenerated electrons and holes, and thereby improves the efficiency of photocatalytic hydrogen production. The effects of inorganic and organic sacrificial reagents on the reaction efficiency and stability were observed and discussed. This work shows that Ni2P as a cocatalyst substituting noble metals can greatly improve the photocatalytic hydrogen production efficiency of γ-Ga2O3 compared to that in pure water and a methanol-water solution.