This work presents the experimental results of a study of the water uptake of two biodegradable bio-composites developed in the laboratory based on polylactic acid (PLA) as a matrix and date palm fibers (DPFs) and Posidonia oceanica (PO) as reinforcement. The choice of the two types of fiber is justified by their biodegradability and their abundance in nature in many countries, particularly Tunisia. Therefore, to optimize the development of these biocomposites, it is essential to understand their durability, particularly their resistance to moisture. Indeed, low water uptake enables the use of these bio-composites in moisture-prone environments while ensuring long-term durability. Low water absorption also ensures bio-composite durability. The procedure consists of monitoring the absorbed mass of each bio-composite over time, until water saturation is reached. This involves regularly weighing standardized samples of these bio-composites after they have been immersed in distilled water at room temperature. The results show that the higher the fiber content of the bio-composite, the more sensitive it becomes to moisture. Furthermore, PLA/DPFs bio-composites are more resistant to moisture than PLA/PO bio-composites for the same fiber content.