Abstract: The effects of multipass friction stir processing (FSP) and Mg powder addition on the different microstructure parts, including the stir zone (SZ), heat-affected zone (HAZ), and thermomechanically affected zone (TMAZ) of Al 1050 alloy were investigated. Microstructural observations revealed that with the increase in the number of FSP passes, the grain size of the SZ decreased in the non-composite and composite samples, whereas that of the TMAZ and HAZ increased in the non-composite sample. Furthermore, the addition of Mg powder resulted in considerable grain refinement, and increasing the number of the FSP passes resulted in a more uniform distribution of Al–Mg intermetallic compounds in the in-situ composite sample. Results of the tensile test showed that the non-composite sample that underwent four passes of FSP exhibited a higher elongation percentage and a ductile fracture in comparison with those of the base metal and the composite sample. However, this sample exhibited a brittle fracture and a higher tensile strength in comparison with the base metal and the non-composite sample. The fabrication of composite samples resulted in a remarkable enhancement in hardness in comparison with the base metal and the non-composite samples that underwent FSP.