In this study, post-weld friction stir processing (FSP) was applied to eliminate the grain coarsening disadvantages encountered in the heat-affected zone when joining AA5754 aluminum alloy with Gas Tungsten Arc Welding (GTAW). As a result of welding in two passes and with the low heat input by selecting the appropriate welding parameters, all welding seams were produced without macro defects. Despite all these precautions, grain coarsening has occurred in the Heat Affected Zone (HAZ), as expected. Significant amounts of grain refinement were detected as a result of the FSP applied to the regions with grain coarsening. Consequently, the grain refinement in the HAZ, the tensile strength increased slightly, in contrast with ductility has significantly increased (around from 6% to 19%). In the tensile tests, the rupture occurred in the HAZ in the GTAWed specimens, while the rupture occurred in the base material close to the HAZ in all FSPed specimens. In addition, the high hardness values of HAZ of the GTAWed samples were reduced to the base material hardness values in all FSPed samples, resulting in a more homogeneous hardness distribution.