A bio-based composite material made from wastes from Posidonia oceanica (PO) and sugar cane polyester (SCPES) has been developed in the current work. DSC and TGA have been used to investigate the thermal deterioration of composites, and the results demonstrate an improvement. Using dynamic mechanical analysis (DMA) data, it has been found that the storage modulus (G) increases significantly as the Posidonia oceanica concentration rises. There has been a 60 % increase in stiffness in 40 % composites compared to unfilled materials in tensile test results. Flexural modulus of polyester, when unloaded is more than twice that of polyester, when fully loaded. In addition, Shore D hardness as well as Charpy impact test findings reveals that HDPE's intrinsic high impact energy absorption is intact in HDPE-PO composites, confirming this improvement in mechanical properties. As a result of the low water absorption rate (less than 8 %) and the long immersion time, these composites are assured to preserve their dimensional stability.