<p>Biopolymers offer eco-friendly alternatives to traditional additives for the stabilization of loose sand. This study investigates the effect of two different types of fiber (polypropylene and glass) for the reinforcement of biopolymer-treated sand and improving material behavior from brittle to ductile. Key parameters such as ductility, compressive strength, flexural strength, toughness indices, and residual strength factor were analyzed to evaluate the influence of fiber type, content, and length on the mechanical properties of reinforced treated sand. Also, microstructural characterization of the sand-biopolymer-fiber composite was performed using scanning electron microscopy (SEM) tests. It is revealed that although reinforcement with glass fiber significantly improves bending strength, introducing polypropylene fiber enhances compressive strength, toughness indices, and ductility more effectively.</p>