This study investigates the influence of the plunging force in friction spot stir welding of polycarbonate sheets on the mechanical behavior of the welds. Experimental tests were carried out by varying the tool geometry and the applied plunging force. Mechanical tests based on single-lap shear tests were carried out for mechanical characterization of the welds. Thus, the morphology of the welds was analyzed to clarify the influence of the plunging force on geometry and defects of the welds. According to the achieved results, the control of the plunging force allows improving the mechanical behavior of the welds up to 37 % without requiring for additional energy during the welding process or affecting the process production time. The increase of the weld strength is due to the reduction of porosities developing at the interface between the stirred zone and the surrounding material. However, excessive plunging force results in weaker welds due to excessive thinning of the punch-sided sheet. Under optimal conditions, the shear strength of the welds was 34.5 MPa that yields that of the base material.