Abstract: A 10 µm thick Al foil was utilized as an intermediate layer for resistance spot welding (RSW) of AZ31B magnesium alloy and TA15 titanium alloy sheets, both having a thickness of 2 mm. Advanced analytical techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers hardness testing, and universal electronic testing, were utilized to investigate the effects of the Al foil interlayer on the microstructural morphology, nugget diameter, tensile-shear load, and hardness distribution of the joints. The results indicate that no distinct lack-of-fusion areas are found in the welded joints. As the welding current increases from 10.00 kA to 11.35 kA, the nugget diameter of the joint without Al foil interlayer expands from 6.8 mm to 10.2 mm, and the indentation rate rises from 4.8% to 24.5%. For the joint with the Al foil interlayer, the nugget diameter increases from 6.5 mm to 8.4 mm, and the indentation rate rises from 4.7% to 23.4%. The tensile-shear load initially increased and then decreased for both joint types. At a welding current of 10.35 kA, the tensile-shear loads for the joints without and with the Al foil interlayer reached maximum values of 4.210 kN and 3.357 kN, respectively. The nugget diameter and the indentation rate of the joint with Al foil are smaller than those without Al foil, and the brittle and hard intermetallic compound Al₁₂Mg₁₇ is found at the interface, which will reduce the tensile shear load of the joint.