In this study, metal particles were added during the spot friction welding (SFW) process, a solid state sheet metal joining process, to create a localized metal matrix composite (MMC) for the improvement of lap shear strength in AISI 6111-T4 aluminum alloy sheets. The Ancorsteel® 1000 particles were compressed between the upper and lower sheets and distributed concentrically around the tool axis perpendicular to the plate surface, which formed a localized MMC and were effective as the reinforcement particles in aluminum 6111-T4 alloy sheets. Results revealed that the MMC reinforcement improved the lap shear strength of SFW joints by about 25%. An aluminum-ferrous solid solution was formed around the steel particles along the aluminum matrix interface. The load-deflection curve shows that the steel particle MMC increased both the strength and ductility of SFW joint. This is attributed to two phenomena observed on the failed lap shear tensile specimens with SFW MMC. One is the longer and more torturous crack path, and the other is the secondary crack on steel particle MMC reinforced SFW joints.