Electrochemical reduction of N to NH provides an alternative to the Haber-Bosch process for sustainable, distributed production of NH when powered by renewable electricity. However, the development of such process has been impeded by the lack of efficient electrocatalysts for N reduction. Here we report efficient electroreduction of N to NH on palladium nanoparticles in phosphate buffer solution under ambient conditions, which exhibits high activity and selectivity with an NH yield rate of ~4.5 μg mg h and a Faradaic efficiency of 8.2% at 0.1 V vs. the reversible hydrogen electrode (corresponding to a low overpotential of 56 mV), outperforming other catalysts including gold and platinum. Density functional theory calculations suggest that the unique activity of palladium originates from its balanced hydrogen evolution activity and the Grotthuss-like hydride transfer mechanism on α-palladium hydride that lowers the free energy barrier of N hydrogenation to *N H, the rate-limiting step for NH electrosynthesis.