Abstract: FeCoCrNiMn high entropy alloy (HEA) sheets were welded to ASTM AZ31 magnesium alloy sheets at four different rotation speeds by using friction stir lap welding(FSLW) technology. The mechanical properties were evaluated through tensile lap shear tests. Various characterization techniques were applied to investigate the effect of rotation speed on macrostructure of the AZ31/FeCoCrNiMn joint. Effect of rotation speed on the tensile lap shear fracture characteristics was also investigated. The results indicate that as the rotation speed increased from 1300 r/min to 1600 r/min, the effective sheet thickness of the upper plate on the advancing side and retreating side of the AZ31/FeCoCrNiMn joint decreased from 4372 μm and 4268 μm to 3815 μm and 3802 μm, respectively. At the same time, the number of high-entropy alloy particles distributed in the magnesium side gradually increased. As a result, the joint’s performance and fracture mode changed. As the rotation speed ranging from 1300 r/min to 1500 r/min, the AZ31/FeCoCrNiMn joint fractured near the interface. A considerable amount of AZ31 residue present on the fracture surface of the HEA side, indicating that the fracture partially occurred within the AZ31. The joint exhibited excellent mechanical properties, with the maximum tensile lap shear strength reaching 108.3 MPa, corresponding to 83.4% of that of the AZ31 base material. At the rotation speed of 1600 r/min, the fracture extended upward from the tip of the hook structure and propagated through the AZ31 plate underloading. Residue HEA particles were observed on the fracture surface, suggesting that these particles provided a route for crack propagation. Consequently, the joint exhibited poor mechanical properties with a maximum tensile lap shear strength of only 57.5 MPa.