In this work, we investigate the effect of the Posidonia Oceanica fiber (POF) rate and matrix type on the mechanical properties and water absorption behavior of the produced composites by compression molding process. The obtained results show that the stiffness, strength and hardness of unsaturated polyester resin (UPR)/POF composite increase with POF reinforcement rates. Further, the choice of matrix plays in important role on the flexural properties of the composites. At equal percentage of POF (20 wt%), UPR/20POF composite shows the best flexural modulus compared to Epoxy/20POF composite. By contrast, the maximum deflection of UPR/20POF is approximately three times less important than the Epoxy/20POF composite. In addition, in the case of UPR/POF composite, water absorption tests revealed that the percentage of water content was found to increase with POF content. It was observed that the water absorption pattern of the all formulation of the composites was found to approach the Fickian diffusion behavior. It was also proven that the moisture resistance of UPR/20POF composite is greater than Epoxy/20POF composite. Compared to literature, composite reinforced with higher load of POF (UPR/30POF) exhibits higher dimensional stability, which explains their importance when it will be used for application in humid environment.