The electrochemical reduction of nitrate to ammonia is considered a feasible strategy to address the imbalance of the global nitrogen cycle by eliminating N-containing pollutants in wastewater while producing ammonia in a sustainable way. In this work, we systematically investigate the reduction of nitrate to ammonia on p-block indium (In), noble metal ruthenium (Ru), and their alloys (InRu3, In3Ru). We found that pure In and Ru candidates exhibit various performances for nitrate reduction with low limiting potentials for the potential-determining step of −0.31 V and −0.14 V, respectively. However, the incorporation of In into Ru to create alloys leads to excellent activity for the reduction of nitrate to ammonia. Ammonia production on InRu3 and In3Ru alloys demonstrates higher catalytic activity and selectivity as the byproducts of NO2 and NO can be inhibited, while the parasitic hydrogen evolution reaction (HER) can also be significantly suppressed. Our work not only proposes efficient electrocatalysts for nitrate reduction but also provides a new strategy for the rational design of alloy catalysts for improved catalytic performance.