Graphite/aluminum metal matrix composite (MMC) joints are successfully fabricated by friction stir spot welding (FSSW). During lap-joining processes of aluminum alloy sheets by FSSW, graphite/water colloid is applied between the tool shoulder and the upper plate. The Raman spectrum of the FSSW joint confirms that graphite/aluminum MMC is successfully induced in the stir zone. Quasi-static shear tests and micro hardness measurements of the FSSW joint also show that the strength and toughness of the joint are significantly improved by inducing graphite/aluminum MMC in the stir zone. The results of the present study suggest that the mechanical properties of structural components joined by spot welds can be easily enhanced by adopting MMC joints by FSSW, instead of changing the material or increasing the number of joints.