Two-dimensional (2D) layer-structured transition-metal sulfide shows excellent Fenton catalytic properties, which has attracted much attention. However, the release of metal cations from the catalyst induces secondary environmental pollution. In the present study, 3D-constructed NiS assembled by 2D nanosheets was synthesized by a hydrothermal method. The NiS displayed high levels of Fenton catalytic activity for the degradation of rhodamine B (RhB) under a wide pH range from 5.60 to 12.21. However, Ni2+ was released in acidic solutions, and the released Ni2+ decreased with increasing pH. The catalytic mechanisms in acidic and alkaline solutions were explored. The high Fenton catalytic activity benefits from the synergistic effect of the Ni2+ and the S2- at pH = 5.60. At pH = 12.21, the S atom accelerated the electron transfer between the Ni(III) and Ni(II) at the NiS surface. The material kept the same catalytic activity for many cycles. These results demonstrate that NiS can be used as a catalyst for the Fenton process in alkaline conditions.