A free-standing three-dimensional (3D) electrocatalyst is demonstrated as an example of fine-tuning the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities through a simple electrodeposition method. The Ni3S2 nanosheets are electrodeposited onto the highly conducting Au dendrites that are grown over 3D-graphene (3DG) framework. The highest electrochemical surface area has been achieved to enhance exposure of active sites by the integration of hierarchical Au dendrites with a sufficient amount of Ni3S2 nanosheets. The optimized electrocatalyst (3DG–Au-Ni3S2-15c) delivers the current density of 10 mA cm–2 at an overpotential of 140 mV and a lowest Tafel slope of 93 mV dec–1 for HER. The higher HER activity is supported by the smaller charge-transfer resistance of 1.09 Ω, which confirms the faster interfacial electron transfer between the electrode and electrolyte phases. The OER experiments suggest that 3DG–Au-Ni3S2 (15c) has a higher current density of 91.2 mA cm–2 at a potenti...