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The welded joints of reduced activation ferritic/martensitic (RAFM) steel should exhibit superior toughness to ensure that structural components are not prone to fracture due to irradiation embrittlement under long-term neutron irradiation conditions. Friction stir welding (FSW) of RAFM steel was performed, considering the potential advantages of obtaining welded joints with a good balance of strength and toughness. This work systematically investigated the effect of hierarchical martensitic microstructural evolution on the impact toughness of friction stir welded (FSWed) RAFM steel in the as-welded condition and post-weld tempering treatment at 760 °C. Prior austenite reconstruction based on the electron backscatter diffraction (EBSD) technology and precipitate analysis according to the transmission electron microscopy (TEM) data were conducted to study the microstructural evolution comprehensively. Tempering treatment reduces the average hardness of the SZ from 446 HV to 269 HV, but it remains 39 HV higher than that of the base material (BM). The tensile specimens containing whole regions of the welded joint all fracture in the BM. The ultimate tensile strength (UTS) of the tensile specimens containing only the SZ reaches 1033.7 MPa and 799.0 MPa in the as-weld and tempered at 760 °C conditions, respectively, which is greater than the 689.3 MPa of the BM. The FSWed joint of RAFM steel in the as-welded condition has acceptable toughness, and its ductile-brittle transition temperature (DBTT) corresponding to 68 J impact absorbed energy reaches −74.2 °C. Post-weld tempering treatment at 760 °C further reduces the DBTT to −110.7 °C, slightly higher than the −114.8 °C of the BM. The refined grain size is the crucial microstructure that determines the FSWed joint of RAFM steel has better toughness and lower DBTT than fusion welded joints.