Refill Friction Stir Spot Welding (RFSSW) is a subset of friction stir welding. The term "refill" is added to this method due to its ability to eliminate the keyhole defect left behind by conventional friction stir spot welding. The core idea of the present research is to investigate how the application of horizontal ultrasonic vibrations to the refill friction stir spot welding process enhances the mechanical and microstructural properties of 6061-T6 aluminum alloy. To achieve this, a test setup consisting of an RFSSW welding unit and ultrasonic equipment was prepared. The specimens were made from 1-mm-thick 6061-T6 aluminum alloy sheets. The input variables, including ultrasonic waves, rotational speed, and plunge depth, were applied to the 6061-T6 aluminum alloy. The resulting welded samples were evaluated and compared in terms of tensile strength, hardness, and microstructure. The results indicated that increasing ultrasonic power during refill friction stir spot welding led to grain refinement in the weld nugget zone. This refinement contributed to a 23% increase in strength at tool rotational speeds ranging from 1200 to 1400 rpm. However, at a higher rotational speed of 1800 rpm, the strength decreased by 25% due to grain coarsening.