As a fascinating metal-free conjugated polymer semiconductor, 2D graphitic carbon nitride (g-C₃N₄) has been widely used as a visible-light-responsive photocatalyst. Generally, g-C₃N₄ exhibits some appealing properties, such as exceptional thermal and chemical stability, appropriate band structure, as well as low cost. However, the pristine g-C₃N₄ is limited by the fast recombination of the photoinduced electron-hole pairs, limited surface area, and insufficient sunlight absorption. Very recently, the construction of 2D/2D heterojunctions has attracted widespread attention owing to their distinct advantages, including the intimate interface, high surface area, suitable band potential and so forth. Therefore, this review summarized the latest progress in the coupling g-C₃N₄ with other 2D nanomaterial systems, including 2D graphene-like materials, 2D transition metal dichalcogenides, 2D metal oxides, 2D bismuth-based semiconductors and 2D phosphorous. Furthermore, the photocatalytic mechanism of g-C₃N₄ based 2D/2D heterojunctions was systematically studied and their corresponding charge-transfer pathways on the interface surface were elaborated. In addition, the photocatalytic applications of the g-C₃N₄ based 2D/2D composites in the realm of hydrogen generation, carbon dioxide reduction, pollutant degradation, and photocatalytic disinfection were also reviewed. Finally, the current challenges faced by the formation of g-C₃N₄ based 2D/2D composite and the opportunities for further development are also proposed.