A voltammetric method has been developed for the determination of trace amounts of Se (IV) using a carbon paste electrode modified with mercury and copper. The conditions for the formation of the modified electrode surface and the electrochemical determination of selenium were investigated. It was established that the optimum formation of the copper amalgam was achieved at a potential of 0.34 V, while the optimum accumulation time was 10 s. A 0.05 M formic acid solution was selected as the supporting electrolyte. The mechanism of the Se (IV) electrode process on the Hg/Cu/CPE surface was investigated, and the main electrochemical characteristics were determined. The proposed electrode provided a linear relationship between the analytical signal and selenium concentration in the range of 0.05–30.0 μg/mL, with a correlation coefficient of 0.996. The detection limit for Se (IV) was 0.05 μg/mL. The effect of accompanying ions on the determination of selenium was also studied. The results showed that the proposed method can be used for the determination of Se (IV) in complex multicomponent systems, including samples containing zinc compounds and other accompanying elements.