We report a simple method of accumulation and electrochemical determination of copper(II) ions using a carbon paste electrode modified with a nanocomposite of Fe3O4/eggshell and carbon nanotubes in aqueous samples. Under stirring, Cu(II) was deposited on the modified electrode and the electrochemical response was amplified. The accumulated Cu(II) on the electrode showed a voltammetric peak at a potential about –0.1 V in 0.2 M HCl solution, which could be used for the measurement of Cu(II). Under optimal conditions, Cu(II) could be detected in the range from 0.5 to 310 ng mL–1 with a detection limit of 0.033 ng mL–1. In particular, with the use of the reduction peak of Cu(II), the modified electrode exhibits excellent performance for Cu(II) determination even in the presence of interference ions. The regeneration of the electrode is facile with good reproducibility. The electrochemical system was applied to analyze Cu(II) in drinking water, wastewater, hair samples, and certified reference materials. In addition to Fourier transform infrared spectroscopy (FT-IR) and vibrating sample magnetometer (VSM) for the characterization of the prepared magnetic nanocomposite, the morphology and structure of Fe3O4/eggshell were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD).