Posidonia oceanica is an abundant marine plant in the Tunisian seas. The waste of this plant is rejected by the sea in the form of balls or leaves and accumulates in significant quantities on the coasts each summer season. The valorisation of this waste is therefore an interesting avenue of research for the production of new bio-based products or materials. This chapter provides an overview of the overall of Posidonia as an annual marine plant, its chemical composition, its multi-scale structure (inspired by the organisation of plants and wood) and its valorisation into nanomaterials. General information on the subject will be presented (for non-expert readers), as well as specific data from recent scientific literature (for expert readers).