Different types of plant waste are available in Tunisia, and their exploitation and possible recovery represent an economic and environmental interest. Among them, Posidonia oceanica is a marine plant abundant on the Tunisian coast. The Posidonia oceanica wastes are rejected on the beaches in huge quantities every year, and these beaches must therefore be cleaned each summer season. The wastes of Posidonia, consisting of a large percentage of cellulose, represent an important source of cellulose fibers at low cost. This chapter presents the derivative cellulosic products obtained from Posidonia oceanica and its applications. These studies serve two important purposes: First, they enable the efficient use of waste materials, and second, they lead to the production of derivatives that are significantly more cost-effective than those currently on the market. This is particularly significant since these derivatives are currently imported from developed nations.