Recently, the use of resistance welding in the joining of polymeric materials and polymer composites in the aviation, marine, and automotive industries has been gradually increasing. However, challenges continue in regard to improving product quality as affected by several parameters. This study focused on the effects of wire geometries and wire diameters on the characteristics of polypropylene (PP) sheets united by resistance implant welding. Here, PP sheets manufactured by the injection molding method were joined via the resistance upset welding method with three different wire geometries, including spiral, wavy, and M-shaped, and two different wire diameters of 0.3 and 0.5 mm. Mechanical properties, morphological properties, and heating characteristics were examined. The results showed that wire diameter and wire geometry affected amount of heat formation in the weld zone and, therefore, weld performance in terms of the mechanical properties. The results obtained from the study can be used as a reference in similar applications.