For the first time, octahedral NiGa2O4 nanocrystals having reactive pH-dependent {111} facets are synthesized through a facile hydrothermal route without using any template or organic surfactant. The {111} facets of octahedral NiGa2O4 display clearly enhanced photocatalytic generation of hydrogen and oxygen from water splitting and good photocatalytic stability. Density functional calculations suggest that mixed statistically occupied Ga/Ni (fourfold- and sixfold-coordinated Ga/Ni) are most likely to be exposed at the (111) surface of NiGa2O4, which is very favorable for enhancing the photocatalytic activities, and the photoelectrochemical properties show that the NiGa2O4 octahedron displays a better photocurrent than NiGa2O4nanorods with the [100] growth direction. The transient photocurrent decay scan results demonstrate that the NiGa2O4 octahedron exposed {111} facet electrode exhibits a transient decay time of 4 s, whereas this time is only 2 s for NiGa2O4nanorod electrodes with the [100] growth direction. This longer transient decay time indicates that the charge-carrier recombination rate is lower in the NiGa2O4octahedron electrode, which will contribute to the enhancement of the photocatalytic activity. The present study also demonstrates that designing nanostructures with the appropriate morphology and surface structures is a feasible approach for enhancing the photoexcited charge-transfer lifetime and developing highly active semiconductor photocatalysts.