One of the most common species that are native to the Mediterranean Sea is the seagrass Posidonia oceanica (L.). The leaves of the P. oceanica plant are likely composed of seven bundles, each measuring 8–11 mm in width and 20–80 cm in length. Rhizomes range in thickness from 5 to 10 mm and carry the roots that connect the plant to the substrate and allow nutrients to be absorbed from the sediment. P. oceanica leaves have been tried for a variety of traditional purposes. For example, they were utilized as a material to make mattresses. Farmers along the Mediterranean coasts have long utilized the dead leaves of P. oceanica as compost. Over the last fifty years, frequent studies have been conducted in Italy, Tunisia, and Greece to make Posidonia-based compost. Nevertheless, multiple studies have been conducted on the dignification of P. oceanica in order to create cellulose and convert it into other derivatives. This chapter describes the chemical composition and pulping of fiber derived from P. oceanica.