This study utilizes a face-centered central composite response surface design of experiments to determine the effects of thread pitch when friction stir welding high-density polyethylene. The tool pin thread pitch, along with rotation and traversing speed, was varied so that models of the maximum tensile strength, tool temperature, and tool forces could be analyzed. Coarser thread pitches facilitated higher tensile strength than finer threads due to greater material velocity and overlap between the stir zone and base material. In the tested range, the thread pitch provided a 6% linear contribution to ultimate tensile strength, whereas welds with coarse threads had on average a 2.83 MPa increase in tensile strength over the fine thread tool. The greater circumferential pin surface area of fine threads caused a greater increase in tool temperature, though this did not correlate to stronger welds. Ultimately, the most interdiffusion across the polymer joint occurred with the coarser thread pitch and slow traverse speed due to prolonged joint exposure to the molten polymer weld pool.