Friction stir spot welding (FSSW), as a solid state joining process, shows great potential to be a replacement of single-point joining processes. However, the possible application of FSSW in polymers needs more attention. In the present paper, experimental studies have been conducted to explore the feasibility of welding polymers by FSSW. The investigation is applied on the most popular polymer, the polyethylene (PE), under different operating conditions. Plunging depth, and dwell time at two different tool rotational speeds have been taken as varying parameters. The effect of the welding parameters on the strength of welded joints has been determined by lap shear test. The temperature in the welding material zone during dwell time has also been recorded to investigate the effect of the duration of dwell time on the heat generation during welding. The results confirm that FSSW can be applied to PE and that the tool rotational speed, plunging depth, and dwell time are important in the joint formation and its strength.