In the current investigation, seeds of the Foeniculum vulgare plant were acquired from the KSA, Tabuk City, and its powder was chemically modified by four different reagents. The four resulting samples were identified by SEM and IR techniques and inspected for methylene blue (MB) dye adsorption. The influence of MB concentration, adsorbent dose, solution pH, temperature, and contact time on adsorption performance was examined. This adsorption's kinetic, isothermal, and thermodynamic parameters were correspondingly studied. The idyllic adsorbent was established to be the modified sample with oxalic acid, which removed 90.15 % of the MB dye. It was also shown that the MB adsorption matched well with the kinetically 2nd -order model. The findings of the experiment fit better with the model of Langmuir isotherm than the other models. The negative values of each ΔG°, ΔH°, and ΔS° indicate the adsorption of MB is an exothermic and spontaneous process. The very law value of adsorption enthalpy proves the physisorption adsorption. The very high maximum adsorption capacity (qmax) values of this adsorption (892.857, 684.932, 487.805 mg g−1), the stability and availability and of this oxalic acid modified adsorbent suggest it will be highly valued in the field of water decontamination.