Joining of lightweight dissimilar materials by friction stir spot welding (FSSW) provides numerous advantages over conventional welding methods such as resistance spot welding and self-piercing rivets. However, the keyhole left behind by the tool is a major drawback in FSSW, as it acts as a stress concentration factor and initiates corrosion. In this paper, the keyhole is avoided in FSSW of Al 5754 and 6061 alloys using pinless tools. The process parameters such as tool rotational speed, plunging speed, dwell time, plunge depth, and shoulder diameter were optimized using response surface methodology with Box–Behnken design. The results showed that shoulder diameter was the most crucial factor that affected the joint strength. The micro-hardness analysis indicated the strengthening of the thermo-mechanically affected zone. SEM and EBSD techniques were employed to analyze the microstructure of the weld joint. © 2020, ASM International.