In the present study, friction stir welding (FSW) was employed to join sheets of EN AW 7075 alloy. Although FSW could refine the microstructure in the stir zone (SZ), the coarsening of precipitates resulted in the deterioration of mechanical properties. Different postweld heat treatments (PWHTs) were utilized to improve the mechanical behavior of friction-stir-welded joints. The effect of various quench media in the PWHT on the mechanical properties and microstructure of the EN AW 7075 joint was studied using tensile testing and hardness measurements. It was found that regardless of the quench media, PWHTs led to abnormal grain growth (AGG) in the SZ. Higher cooling rates applied via water quenching from solution heat treatment caused the formation of a higher degree of supersaturation compared to that of air-cooled counterpart. The high degree of supersaturation in the water quenched specimen resulted in the formation of fine and dispersed precipitates in the subsequent aging treatment and eventually improvement in the mechanical properties. Degradation of mechanical behavior in the air-cooled specimens was attributed to the formation of coarse precipitates and precipitate-free zones (PFZs) along and in the vicinity of grain boundaries, respectively.