<p>Interfacial evolution and bonding mechanism of reduced activation ferritic/martensitic (RAFM) steel were systematically investigated through a series of hot compression tests conducted at various strains (0.15–0.8), strain rates (0.001–1 s⁻¹), and temperatures (950–1050 °C). Interfacial microstructural analysis revealed that plastic deformation of surface asperities effectively removes interfacial voids, and the evolution of dynamic recrystallization (DRX) aids in achieving a joint characterized by homogeneously refined microstructure and adequate interfacial grain boundary (IGB) migration. Electron backscattered diffraction analysis demonstrated that the continuous dynamic recrystallization, characterized by progressive subgrain rotation, is the prevailing DRX nucleation mechanism in RAFM steel during hot compression bonding. During DRX evolution, emerging DRX grains in the interfacial region expand into adjacent areas, transforming T-type triple junction grain boundaries into equal form, and resulting in a serrated and intricate interface. Elevated temperatures and strains, coupled with reduced strain rates, augment DRX grain nucleation and IGB migration, thus enhancing RAFM joint quality with regard to the interface bonding ratio and the interface migration ratio.</p>