Nitrate reduction was investigated on a polycrystalline Pt electrode modified with p-block metals, such as indium, gallium, thallium, tin, lead, arsenic, antimony, bismuth, zinc, and cadmium, in both perchloric and sulfuric acid solution. Online electrochemical mass spectrometry was applied to detect volatile products on the electrode surface. In perchloric acid, tin, cadmium, indium, and gallium were found as active promoters for nitrate reduction, and N2O was detected as the main product for cadmium- and indium-modified Pt electrode, whereas the tin-modified electrode produced both NO and N2O. Lead cations inhibited nitrate reduction, but N2O was still formed selectively. In sulfuric acid, tin was an active promoter and showed the same coverage dependence for the product distribution as was reported previously for perchloric acid. Thallium performed similarly to germanium in that both metal cations only promoted nitrate reduction in sulfuric acid but not in perchloric acid, which could be attributed to their effect on disordering the bisulfate adsorption on Pt to recover active sites for nitrate reduction. Arsenic, antimony, and bismuth exhibited a similar effect on nitrate reduction on Pt sites in both acid electrolytes; they inhibited nitrate reduction on free Pt sites in the low-potential range. Among the p-block metals that were investigated, tin was the most active promoter for nitrate reduction, considering both the anion effect and the product distribution. DFT calculations were performed to gain insight into the promoting role of the two most active promoters, tin and indium.