Owing to the demand for chemical analysis of low concentrations of the Brilliant Green (BG) dye released into the environment, molecularly imprinted polymer (MIP) materials have been investigated for their use in SPME preconcentration devices. Thin coatings of MIP on solid supports allow fast adsorption of the analyte and higher selectivity of adsorption. Biosourced fibers of Posidonia oceanica (Pos) were taken as a the solid support to which a thin film of MIP was chemically attached. The material synthesis involved two steps: chemical grafting of methacrylate groups onto Pos by a silane coupling, and copolymerization with the free functional monomer and cross-linker in the presence of BG. Polymerization of the functional monomers methacrylic acid and acrylamide yielded the materials PosMAA@MIP and PosAA@MIP respectively. The strength of their interaction with BG and the stoichiometry of the pre-polymerization complex were assessed by $^1$H NMR experiments in DMSO-d6 solution. Characterization by IR spectroscopy and thermogravimetric analysis validated the successful syntheses. The points of zero charge were at pH 3.31 and 4.50 for PosMAA@MIP and PosAA@MIP, respectively, influencing the optimal adsorption of BG at pH 4. Kinetic studies revealed the rapid adsorption of more than 90% of the BG within 20 min. All MIPs exhibited higher adsorption capacity for BG compared to non-imprinted materials. PosAA@MIP showed the highest adsorption efficiency and selectivity, with an imprinting factor of 1.9. The thermodynamics of adsorption was studied by fitting the Langmuir–Volmer and Volmer models to the experimental data, from which the selective adsorption capacities of BG on PosMAA@MIP and PosAA@MIP were 289 and 313 µmol·g$^{−1}$, respectively. Additionally, PosAA@MIP exhibited higher extraction efficiency for BG than for its metabolites eosin and methylene blue, underscoring its superior ability to selectively capture target molecules