This article presents the results on using a new electrochemical sensor modified with arenediazonium salts (MGE–Cu–COOH) to determine selenium in biological fluids (blood serum). It was shown that the sensitivity of the determination of selenite ions using this modified electrode is higher compared to that of the unmodified one (MGE–Cu). The effect of the concentration of arenediazonium tosylates on various substituents was studied, and the conditions for the production of the electrochemical sensor were developed. The surface of the modified electrode was investigated using SEM. The increase in the effective surface area of the electrode was found to be due to the formation of a system of ultramicroelectrodes. In biological samples, the LOD and LOQ of selenium were established at the levels of 0.057 and 0.166 μg l−1, respectively. Finally, the correctness of the results on selenium determination in real samples by the “input-found” method, which correlates well with the known value, was verified.