The fatigue performance of dissimilar AA6013/Ti−6Al−4V friction-stir lap welds produced under a zero-penetration condition was evaluated. The welded joints were assembled by placing the aluminum alloy workpiece on the upper part of the dissimilar joints and plunging the welding tool solely into the aluminum alloy part, with the distance between the probe tip and dissimilar interface kept at about 50 μm. The fatigue strength of the welded joints was essentially lower than that of the aluminum base material. This observation was attributed to the easy nucleation of the fatigue crack at the end section of the unwelded interface. Accordingly, the weld fatigue behavior was governed by crack propagation. Nevertheless, the welds typically failed through the aluminum part, and thus the intermetallic layer evolved at the dissimilar interface was not a critical issue. Therefore, it was concluded that the zero-penetration approach is feasible for dissimilar welding.