The fatigue behavior of friction stir spot welds in magnesium AZ31 alloy is experimentally investigated and modeled. The friction stir spot welds employed in this study are representative of preliminary welds made in developing the joining process for potential use in automobile manufacturing. Load control cyclic tests were conducted on single weld lap-shear coupons to determine fatigue life properties. Optical fractography of the failed fatigue coupons revealed that fatigue cracks initiated from the interfacial “hook” and eventually failed by either nugget pullout or full width separation, depending on the cyclic load amplitude. The failure modes of the magnesium AZ31 alloy were similar to the aluminum alloys of comparable friction stir spot welds. To predict the fatigue life of the lap-joint coupons, a crack growth modeling approach based on a kinked crack stress intensity solution was used. The fatigue model predictions compared well to the experimental fatigue life results, despite an approximate stress intensity factor solution for this weld geometry. The experiments and modeling conducted in this study suggest that the size of the interfacial hook, which comes about from the speed, depth of plunge, dwell time, and tool configuration of the friction stir spot weld process, is a major contributor to the fatigue life of the joint.