A highly sensitive and selective electrochemical sensor for Cu²⁺ ions was fabricated by modifying a glassy carbon electrode (GCE) with a CdS/rGO nanocomposite. The structural and morphological characteristics of the synthesized materials were thoroughly investigated using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. These analyses confirmed the synthesis of the CdS/rGO nanocomposite via a hydrothermal synthesis route. The electrochemical performance of the modified electrode was systematically evaluated using cyclic voltammetry (CV), differential pulse anodic stripping voltammetry (DPASV), and electrochemical impedance spectroscopy (EIS). The CdS/rGO-modified electrode exhibited enhanced electrocatalytic activity, with well-defined anodic and cathodic peak currents that demonstrated a linear relationship with the square root of the scan rate over the range of 50–250 mV s⁻¹. Under optimized conditions, the sensor demonstrated appropriate analytical performance over a Cu²⁺ concentration range of 10–100 nM, with a remarkably low limit of detection (LOD) of 2.83 nM. Furthermore, the sensor exhibited good selectivity toward Cu²⁺ in the presence of various interfering ions, along with good reproducibility and long-term operational stability. Its practical applicability was further demonstrated through the successful determination of Cu²⁺ ions in water samples and complex matrices, yielding satisfactory recovery values. These results highlight the potential of the CdS/rGO-based sensor as a reliable and sensitive platform for the practical electrochemical detection of Cu²⁺ ions.