This article describes the synthesis of nanosilica-cysteine composite (SiO2-Cys) and its application as a sorbent for arsenic(III) removal from different aqueous media. Attenuated total reflectance-Fourier-transform infrared spectroscopy, scanning and transmission electron microscopy, X-ray diffraction, and thermogravimetric analysis were applied to characterize SiO2-Cys. Using the batch technique, sorption of As(III) ion by SiO2-Cys was studied, and the effects of pH, sorbent dosage, temperature, initial concentration, and contact time were all taken into consideration. According to kinetic studies, the pseudo-second-order equation adequately described the sorption of the As(III) ion. The spontaneity of the sorption process on SiO2-Cys is suggested by the negative values of Gibbs free energy (G°). Positive values of enthalpy (ΔH°) indicate the endothermic adsorption process and the positive values of entropy (ΔS°) for As(III) ions adsorption imply that the adsorption involves increasing randomness. The Langmuir model, which has a maximum sorption capacity for SiO2-Cys of (66.67 mg/g) at 25°C, provided a better fit to the sorption isotherm.