Lignocellulosic fibersLignocellulosic fibers are increasingly employed in various industrial sectors due to their low cost, mechanical strength, and biodegradability. This study investigates polyester matrix compositesComposites reinforced with different concentrations (10%, 20%, and 30%) of fibers from the stem of Euterpe oleraceaEuterpe oleracea (açaí palm). The compositesComposites were characterized through Charpy impact and tensile tests, Fourier-transform infrared spectroscopy (FTIRFourier-Transform Infrared Spectroscopy (FTIR)), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The incorporation of açaí fibers substantially enhanced the mechanical performance, with increases of up to 2301% in impact resistance, 423% in ultimate tensile strength, 111% in elongation at break, 432% in elastic modulus, and 1004% in toughness. Conversely, thermal stability decreased, with the peak degradation temperature reduced from 382 °C for neat polyester to 332–373 °C for the fiber-reinforced compositesComposites. SEM images revealed weak fiber–matrix adhesion, likely due to the presence of surface waxes and natural impurities, although the fibers effectively impeded crack propagation, enhancing toughness. These findings demonstrate that açaí palm fibers are promising reinforcements for polyester compositesPolyester composites, improving mechanical behavior while slightly compromising thermal stability. The results highlight the potential of lignocellulosic residues as sustainable alternatives for developing high-performance, eco-friendly composite materialsComposite materials.