Due to their unique physicochemical properties, two-dimensional (2D) nanomaterials have shown unprecedented potential in the field of flame retardant polymers. This paper provided a systematic review of the flame-retardant mechanisms of typical 2D materials, such as graphene, 2D transition metal carbonitrides, hexagonal boron nitride,2D metal-organic frameworks, and layered double hydroxides, in polymer matrices. The effects of two-dimensional materials on the mechanical properties of polymers and their multifunctional synergistic effects were summarized in this review. By analyzing the structure-activity relationship between structural properties and flame retardant behavior of 2D materials, the unique mechanism of action in physical barrier construction, catalytic carbonization, and free radical quenching was revealed. The interface control strategy in complex application scenarios was further discussed. Finally, the 2D materials were summarized from the aspects of thermal management, environmental tolerance, intelligent response, and integration of electromagnetic function and flame retardant properties, which provides a research scheme for the development of flame retardant multifunctional materials.