In this study, Fe3O4@SiO2 core-shell nanoparticles functionalized with theophylline molecules were synthesized and used as an effective and strong nanoadsorbent for the removal of divalent copper ions from aqueous solutions. The synthetic and structural steps of the nanoadsorbent, morphology and particle size were investigated using techniques such as Fourier transform infrared spectroscopy, X-ray diffraction, nitrogen gas adsorption-desorption, vibrating sample magnetometer, thermal analysis, scanning electron microscopy, transmission electron microscopy and particle size distribution. In order to evaluate the adsorption performance of nanadosorbent in removing copper ions, initially the optimization of nanoadsorbent dosage, solution pH, initial copper ion concentration and nanoadsorbent contact time were investigated and the results showed the best adsorption performance in the presence of 21 mg of synthetic nanoadsorbent, pH 7 in 75 mL of solution with initial concentration of 0.55 mmol/L at ambient temperature and contact time of 20 min. The adsorption isotherm followed the Langmuir model which resulted in a maximum adsorption capacity of copper ions of 134.7 mg/g. The calculated parameters indicated that the kinetic adsorption data were in well agreement with the pseudo-second-order kinetic adsorption model. This data confirmed that the chemisorption was the main rate-determining step. This synthetic nanoadsorbent is recovered with a magnetic field and reused in consecutive adsorption-desorption cycles for seven times without a noticeable decrease in adsorption activity.