This paper presents an optimization of the mechanical properties of cement–Posidonia composite by means of experimental characterization. We are interested in producing composite of cement reinforced by Posidonia raw fibers. The ratio of fibers and water-to-cement mass is varied to determine its effect on the mechanical properties of the cement–Posidonia composite, particularly, its resistance to fracture. Three point bending and compression tests were carried out to study the mechanical properties of the composite. Scanning electron microscopy was used to examine the surface of the tested samples. The experimental investigation shows an improvement of flexural strength for a ratio W/C equal to 0.5 and fiber content equal to 10 vol%.