Refill friction stir spot welding (RFSSW) is used to weld 3.2-mm-thick 2060 aluminum alloy sheets. In this study, we investigated the formation, microstructures and mechanical properties of the RFSSW joint. The results show that increasing the rotational speed is beneficial to eliminating annular groove but increasing the flash. The hook of the joint at each rotational speed is downward bending. The void at the bottom of the sleeve-affected zone (SAZ) and incomplete bonding at the lap interface initially increase and then decrease with the rotational speed increments from 2000 to 2600 rpm. The microstructure distribution at the bottom of the stir zone (SZ) is similar to that at the interface of the SAZ/thermo-mechanically affected zone (TMAZ), including SZ, TMAZ, heat-affected zone, and the base material. The microhardness values of the joint gradually decrease along the sheet thickness at each welding condition. Furthermore, the microhardness at the SAZ/TMAZ interface initially increases to the maximum at 2400 rpm and then decrease to the minimum at 2600 rpm. Moreover, the tensile-shear failure load of the joint initially increases and then decreases with increasing rotational speeds. The fracture positions of tensile-shear specimens are related to microstructure distribution, microhardness, material flow, and welding defects.