Molecular polyanion ammonium heptamolybdatetetrahydrate [(NH₄)₆(Mo₇O₂₄)]·4H₂O (AHMT) was used to remove methylene blue (MB) dye from an aqueous solution through adsorption technique. Due to an electrostatic interaction of MB cation and AHMT anion adsorption take place. AHMT and MB adsorbed AHMT named as MB-AHMT was characterized by FT-IR, PXRD, SEM and SEM-EDX techniques. Specific surface area (22.175 m² g⁻¹) and pore volume (0.059 cm³ g⁻¹) of AHMT was analyzed by BET analysis. The thermal stability and thermal degradation kinetics of adsorbent were assessed by the thermogravimetric curves recorded at different heating rates. The adsorption equilibrium and kinetic studies indicate that adsorption follows with pseudo first order (R > 0.9998) rather than pseudo second order. Temperature variation study indicates that adsorption was spontaneous, exothermic and entropy increased processes. Experimental data fitted well with Langmuir rather than Freundlich isotherm with maximum adsorption capacity of 877.19 mg g⁻¹. Excess AHMT was recovered as salt of TBA-AHMT which was characterized by FT-IR spectroscopy. The investigations indicate that AHMT may be a prospective material for the removal of MB dye from polluted water.