The aim of this study was to synthesize magnetic Fe3O4 chitosan nanoparticles (m-Fe3O4-CNs) by ionic gelation method and use them as adsorbent for the removal of Bromothymol Blue (BB) from aqueous solutions. Also, the effect of various parameters on the preparation of m-Fe3O4-CNs was investigated in this study. The nanoparticles were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), Fourier transform infrared (FTIR) spectroscopy and vibrating sample magnetometry (VSM). Adsorption of BB on m-Fe3O4-CNs was studied in a batch reactor at different experimental conditions such as adsorbent dosage, pH, contact time, initial BB concentration and temperature. Kinetic behaviors, equilibrium isotherms and thermodynamics of the adsorption process were investigated in detail. The Langmuir adsorption isotherm model and pseudo-second-order kinetic model well fitted the adsorption experimental results. The thermodynamic parameters showed that the adsorption was a spontaneous, favorable and exothermic process in nature.