This study investigates the mechanical, thermal, and morphological properties of high-density polyethylene (HDPE) composites reinforced with walnut shell and nano clay. Tensile and three-point bending tests reveal that nano clay improves tensile strength through enhanced interfacial interaction, with peak values reaching 26 MPa for pure HDPE and 47.9 MPa for HDPE containing 20 % walnut shell and 1 % nano clay. Nano clay also acts as a nucleating agent, increasing the elastic modulus from 601.8 MPa in pure HDPE to 919.6 MPa in HDPE with 20 % walnut shell and 1 % nano clay. Charpy impact tests show strength gains with nano clay, with impact values increasing from 23.82 kJ/m² in HDPE with 20 % walnut shell to 28.42 kJ/m² when 1 % nano clay is added. Hardness measurements increase with fillers, from 67 Shore D for pure HDPE to 75 Shore D for HDPE with both 20 % walnut shell and 1 % nano clay. Water absorption reaches 0.75 % in composites with both fillers, while surface gloss, initially reduced by walnut shell to 7 GU, increases to 21 GU with the addition of nano clay. FT-IR spectra confirm integration of all components, and DSC analysis indicates that nano clay influences crystallinity, altering melting and crystallization rates. TGA results show enhanced thermal stability and higher activation energy with nano clay. Morphological analysis via TEM and SEM demonstrates that nano clay clusters improve fracture toughness, while walnut shell alters surface morphology and porosity. These findings suggest strong potential for industrial applications.