This article is devoted to the study of the effect of date palm seed (DPS) powder on the properties of recycled linear low-density polyethylene (RLLDPE) matrix. A series of composites was developed with a different filler loading varying from 0 to 30 wt% using twin screw extruder followed by compression molding. The result analysis showed that the addition of DPS decreases the percentage of crystallinity of RLLDPE (from 49.4 to 29.8). However, there were no changes in the heat of melting (Tm) and crystallization (Tcry) temperature which was determined by differential scanning calorimeter. Thermogravimetric analysis showed a decrease in thermal stability for RLLDPE with an increase in filler loading which was supported by the change in activation energy (700–456 kJ mol−1) calculated using Broido’s plot. Due to the poor fiber–matrix adhesion, the mechanical properties of the DPS/RLLDPE decreased as function of the filler content and the result was corroborated by the images taken by scanning electron microscopy.