This study aims to develop sustainable and functional textile structures by enhancing the valorization of marine and textile waste—specifically Posidonia Oceanica and cotton fibers. Posidonia, a renewable marine biomass with a high cellulose content, was selected for its environmental abundance and potential as an eco-friendly material. The methodology involves morphological and chemical characterization of raw and chemically treated Posidonia fibers, including cationization using polyimethyldiallylammonium-diallylamine chloride (PDDACD), followed by FTIR and microscopy analyses. Results show that Posidonia fibers have comparable chemical composition to cotton but significantly differ in morphology and diameter. The chemical treatment enhanced fiber compatibility, suggesting improved adhesion potential in composite structures. These findings demonstrate the feasibility of using Posidonia as a biodegradable alternative to synthetic fibers, particularly for packaging and reinforcement in composite applications. The study provides original insight into the functional reuse of marine waste through eco-design strategies, contributing to circular economy practices in textile and material engineering.