In this work, an attempt was made to use recycled thermosetting polymer waste (RTPW) to reduce the environmental impacts. The RTPW and high‐density polyethylene (HDPE) were used to fabricate the composites using melt mixing, a twin‐screw extruder, and an injection molding machine. The weight ratios varied between the HDPE/RTPW: 100:0, 95:5, 90:10, and 85:15 and subjected to different thermal property studies. The thermal properties were evaluated by melt flow index (MFI), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), heat deflection temperature (HDT), and VICAT softening temperature (VST). Results showed a noticeable MFI increase from 9.3 to 14.6 g/10 min in HDPE + RTPW (95:5) composite samples. It was ascribed to the presence of RTPW, resulting in reduced viscosity. DSC reported a minor change in the melting temperature of HDPE due to adding 5% RTPW and 6% polyethylene‐grafted maleic anhydride. These results suggested that the HDPE's crystallinity was minimally changed. Similarly, the onset and end‐set temperatures were increased by incorporating RTPW in TGA, highlighting the influence of thermosetting polymers. Then, the HDT and VSTs were improved by adding RTPW. These observations collectively demonstrate that using RTPW enhances the thermal behavior of HDPE composites while promoting environmental sustainability.