On the basis of ultrathin two-dimensional (2D) nickel metal organic framework (UNiMOF) nanoflakes and 2D g-C3N4 nanoflakes, fresh visible-light-driven 2D/2D heterostructure catalysts were designed, which were assembled with a simplistic electrostatic self-assembly process. The photocatalytic performance of UNiMOF/g-C3N4 for H2 production was evaluated in visible light. The hydrogen evolution of 25.0% UNiMOF/g-C3N4 (UNG-25.0) heterojunction was 20.03 μmol h–1, which was greater than that of pure g-C3N4. In addition, the UNG composites also presented more excellent photocatalytic activity than that of bulk NiMOF/g-C3N4 (BNG) hybrid. This enhanced performance may depend on cooperative potentiation between UNiMOF and g-C3N4 efficiently lowering recombination of carriers. This work showed that constructing 2D–2D heterojunctions provides a feasible approach to obtain highly capable catalysts for photocatalytic decomposition of water.