Two-dimensional fence-like Co-doped NiSe₂/C (Co-NiSe₂/C) nanosheets are fabricated via a facile solvothermal method followed by a selenization strategy. The Co-NiSe₂/C nanosheets are composed of interlaced nanorods, which are made up of small nanoparticles with a size of 20 nm. The surfaces of these small nanoparticles are covered in thin carbon layers. The material displays excellent long-term cycle life (306.1 mA h g⁻¹ after 5000 cycles at 5 A g⁻¹), rate capability (260.1 mA h g⁻¹ at 10 A g⁻¹), and full cell performance (269.1 mA h g⁻¹ after 100 cycles at 0.5 A g⁻¹) as an anode for SIBs. Furthermore, the Na storage mechanism and the reaction kinetics are discussed based on ex situ X-ray diffraction analysis, cyclic voltammetry measurements, a galvanostatic intermittent titration technique, and electrochemical impedance spectra. It is demonstrated that Na⁺ diffusion in Co-NiSe₂/C is a dynamic change process and is accompanied by a phase transition during the discharge/charge processes.