Metal selenides have received much attention from the rechargeable battery community owing to their considerable capacity. However, attainment of a long cycle life and fast Na+ storage kinetics are still needed to enable their practical application. Co0.85Se nanosheets prepared using a simple hydrothermal method exhibit a hierarchical porous structure, which is conducive to fast electrolyte transport and rapid adsorption of Na+ ions. Herein, to improve the electronic conductivity of Co0.85Se, reduced graphene oxide (rGO) is introduced to construct a three-dimensional network and achieve fast electronic transport. This defect-controlled Co0.85Se@rGO anode exhibits a highly reversible capacity of 460 mA h g−1 at a current density of 0.5 A g−1. Outstanding durability and remarkable rate stability are also achieved via the synergistic effects of vacancy transport and the heterostructure between Co0.85Se and rGO. Furthermore, the Na+ insertion/extraction mechanism is systematically investigated using ex situ X-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, and a series of electrochemical analyses. These analyses enable detailed characterization of the ionic diffusion kinetics and corresponding components and structural changes. Finally, a first-principles approach is employed to determine the diffusion energy barrier of Na+ ions in perfect CoSe and defective Co0.85Se.