Waste tire rubber (WTR) supplied by a truck tire retreader were processed in an intermeshing co-rotating twin-screw extruder (ICTSE). The extrusion process evaluated the efficiency of the thermomechanical recycling in the devulcanization of WTR rubbers. Samples were prepared by varying the process parameters, the particles sizes and thermoplastics, and the latter was used as devulcanization auxiliary agents. After extrusion, samples were subjected to solvent extraction to determine the soluble fraction (SF). Subsequently, these SF were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The highest SF (29 wt%) was obtained with higher screw rotating speed and with smaller particle size. Higher SF indicated a higher degree of devulcanization. The FTIR and DSC analyses showed that natural rubber was the main rubber extracted from extruded samples. In addition, polypropylene was more effective than low-density polyethylene in the devulcanization process, promoting higher SF.