Abstract: The aluminum/steel vortex flow-based friction stir lap welding technology was used to weld 5083 aluminum alloy and 304 stainless steel. The effect of tool rotation speed on the macroscopic morphology, microstructure and mechanical properties of the joint was investigated, and the relationship between the interface microstructure and the tensile shear properties of the joint was clarified. The experimental results show that the weld surface is well formed at low and medium rotation speeds, and point defects appear on the weld surface at a high rotation speed of 600 r/min. The aluminum/steel lap interface is flat, tightly bonded, and has no hook defects. The interface is metallurgically bonded through intermetallic compounds and diffusion layers, and the elements of the steel side diffuse to the aluminum side across the intermetallic compound layer. At a rotation speed of 200 r/min, a nanoscale intermetallic compound layer is formed on the interface, and the tensile shear ultimate line load of the joint is the highest, reaching 479.5 N/mm. As the rotation speed increases to 400 r/min, the intermetallic compound layer on the interface thickens to about 1.2 ~ 1.3 µm, resulting in a decrease in the tensile shear ultimate line load of the joint to 332.8 N/mm. When the rotation speed continues to increase to 600 r/min, the thickness of the intermediate layer at the aluminum/steel interface is basically unchanged, and the tensile shear ultimate line load of the joint increases to 411.4 N/mm, which may be due to the partial amorphization of the intermetallic compounds in the interlayer.