This work investigates the use of manganese-iron oxide composites (FS/MnO-FeO) supported on pristine fennel seeds (PFS) for the adsorption of methylene blue (MB) dye. Adsorption isotherms, kinetics, thermodynamics and the effect of pH were investigated. The SEM and TEM showed that the surface of PFS and FS/MnO-FeO was rough and porous and the nanoparticles were attached on the PFS surface. The FTIR spectra demonstrated that there was a shift in –OH, –C=O, –COOH and –COC– wavenumbers after modification. This indicates that indeed nanoparticles were attached on the PFS surface. The adsorption kinetics and isotherms suggested that the process occurred on adsorbents heterogeneous surface and involved electrostatic attraction between MB molecules and adsorbents surface. Thermodynamic studies confirmed that the adsorption process was favoured. Additionally, FS/MnO-FeO showed an effective and environmentally friendly adsorbent.