The g-C₃N₄/Cu nanocomposite was prepared by in situ reduction of Cu²⁺ adsorbed on the surface defects of g-C₃N₄, and its tribological properties as an additive were investigated in base oil. Cu nanoparticles can be well distributed on g-C₃N₄ sheets with an average diameter of 20 nm as revealed by X-ray diffraction patterns and transmission electron microscopic analysis. The tribological properties were determined on an UMT-2 ball-on-disk friction and wear tester. The results indicated that the g-C₃N₄ and Cu nanoparticles as additives were able to effectively reduce both the friction and wear of sliding pairs in base oil. Moreover, the base oil with g-C₃N₄/Cu exhibited the best lubricating behavior with the lowest friction coefficient and wear rate compared to g-C₃N₄ or Cu nanoparticles as additives. Based on our characterization of the wear scars by scanning electron microscopy, a surface mapping profiler, and an energy-dispersive spectrometer, the synergistic lubrication mechanism of g-C₃N₄ and Cu nanoparticles were also proposed.