Water pollution and its hazardous effects due to inadequate disposal of toxic arsenic (As) species is a severe environmental concern globally. In the present study, magnetic Fe3O4 nanoparticles (NPs) functionalized with L-cysteine (Fe3O4@Cy) were synthesized by a simple surfactant-assisted solvothermal method followed by a post-treatment process. The surface functionalization of Fe(3)O(4)NPs with L-cysteine (L-Cy) not only improves their stability but also enhances the As adsorption efficiency. The synthesized materials were extensively characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, vibrating sample magnetometer (VSM), thermogravimetric analyzer (TGA), zeta-potential, nitrogen adsorption-desorption, field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) techniques. The mesoporous Fe3O4@Cy exhibited superior adsorption capacities of 20.00 and 34.00 mg/g for arsenite (As(III)) and arsenate (As(V)) ions, respectively. The kinetics and isotherm studies reveal that the adsorption process follows pseudo-second-order and Langmuir isotherm models. Also, from the thermodynamic study, it is clear that the adsorption process is spontaneous and endothermic in nature. FTIR spectroscopy, FE-SEM, energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) techniques provided insights into the adsorption mechanism.