A theory proposed for understanding the alloying effect on the hydrogen overpotential for transition-metal based alloys was further confirmed in Zr alloys from a series of experiments and electronic structure calculations. The hydrogen overpotential was measured of Zr binary alloys containing a constant amount (3 mol%) of 3d, 4d and 5d transition elements. It changed largely with alloying transition elements, M. Such changes in the hydrogen overpotential resembled the characteristic changes observed for the elements, M, to be in the pure metal state. These experimental results could be explained using the calculated ionicity of M in Zr by the DV-X alpha cluster method. All the alloying elements except for Hf, had the negative ionicity due to the electron transfer from the base Zr metal to M. The negatively-charged alloying elements acted as the preferable site for the hydrogen evolution reaction, resulting in the corresponding changes in the hydrogen overpotential with alloying elements. Thus, the proposed theory was found to be valid for Zr alloys.