Superparamagnetic iron oxide nanoparticles (SPIONs) were synthesized by the hydrothermal method and used for effective removal of arsenite, As(III), in their native and alginate beads-encapsulated (SPIONs-Alg) forms. The size of SPIONs was determined as ∼25 nm, and the structural properties of the adsorbents were validated using FTIR and XRD. The magnetization curve had zero coercivity, indicating superparamagnetism. Furthermore, the effects of pH, contact time, temperature, adsorbent dosage, and initial As(III) concentration on removal efficiency were studied. The optimum removal percentages for SPIONs and SPIONs-Alg were 99% and 90%, respectively, at pH 7, 30 °C, and 6.5 mg/L As (III) concentration. The Langmuir isotherm model (R² ≥ 0.97 for SPIONs and R² ≥ 0.99 for SPIONs-Alg) fitted the equilibrium data better than Freundlich. The As(III) adsorption capacity of sorbents was evaluated using the Langmuir adsorption isotherm and found to be 11.89 mg/g and 240.081 mg/g for SPIONs and SPIONs-Alg, respectively. The adsorption kinetic data for both adsorbents showed a better fit to the pseudo-second-order kinetic model (R² ≥ 0.99). The spontaneity of the adsorption process, the endothermic nature of the sorption reaction, and the adsorbents' affinity for As(III) were determined using the negative ΔG, positive ΔH and ΔS values. SPIONs-Alg (1.5 g/l solid-to-liquid S/L ratio) could be collected easily, recovered using 0.1 M NaOH, and reused for five times (sorption ≥ 97%). The feasibility of SPIONs-Alg as a promising adsorbent for removing As(III) from wastewater is clearly validated.