Abstract: The reliable welding of T91 heat-resistant steel to 316L stainless steel is a considerable issue for ensuring the safety in service of ultra-supercritical power generation unit and nuclear fusion reactor, but the high-quality dissimilar joint of these two steels was difficult to be obtained by traditional fusion welding methods. Here we improved the structure–property synergy in a dissimilar joint of T91 steel to 316L steel via friction stir welding. A defect-free joint with a large bonding interface was produced using a small-sized tool under a relatively high welding speed. The bonding interface was involved in a mixing zone with both mechanical mixing and metallurgical bonding. No obvious material softening was detected in the joint except a negligible hardness decline of only HV ~10 in the heat-affected zone of the T91 steel side due to the formation of ferrite phase. The welded joint exhibited an excellent ultimate tensile strength as high as that of the 316L parent metal and a greatly enhanced yield strength on account of the dependable bonding and material renovation in the weld zone. This work recommends a promising technique for producing high-strength weldments of dissimilar nuclear steels.