Friction stir lap welding (FSLW) was adopted to join successfully dissimilar AZ31B Mg alloy and TC4 Ti alloy with Sn foil addition of 30 µm thickness. Interfacial microstructure, tensile shear performances and bonding mechanism of the joints obtained using three different rotation speeds were studied. High-performance FSLW Mg/Ti dissimilar joints with maximum tensile shear strength of 593.3 N/mm were produced at 1180 r/min, and which was mainly attributed to ultrastrong reaction interlayer consisting of 125.9 nm thick (Mg₂Sn+Mg) transition layer and discontinuous (Ti₆Sn₅+Ti₃Al) IMCs layer with 6.58 nm thickness at the interface. The formation of the reaction interlayer was beneficial for high interfacial strength, resulting in significantly improving the joint strength. The fracture of all FSLW joints located on AZ31B Mg stirred zone adjacent to (Mg₂Sn+Mg) transition layer or along the crack propagation direction of the AZ31B/TC4 interface with different fracture mechanisms, and which could be consistent with interfacial microstructure.