In this work, a new friction spot welding process called “Threaded Hole Friction Spot Welding” was introduced to join aluminum and short-carbon-fiber-reinforced polypropylene composite sheets. The new process was based on the filling of the pre-threaded hole on the metallic sheet by the melted polymer. Mechanisms of bonding were investigated using macro/micro structural investigation of the joints. The effects of the tool rotational speed on the mechanical strength and fracture energy of the joints were also studied. The results showed that the hole is completely filled with the melted polymer. Formation of a reaction layer composed mostly of Al, C and O as well as mechanical locking between the threaded hole and the re-solidified polymer inside the hole was effective on the joint strength. Maximum shear-tensile strength of the joints reached to ~80 percent of the strength of the polymeric composite sheet. Moreover, strength and fracture energy of the joints increased with enhancement of the tool rotational speed. Variation of the joint strength was explored at the light of the fracture surface features as well as the hardness of the re-solidified polymer inside the hole.