The electrochemical nitrogen reduction reaction (NRR) under ambient conditions is a promising alternative to the traditional energy-intensive Haber–Bosch process to produce NH3. The challenge is to achieve a sufficient energy efficiency, yield rate, and selectivity to make the process practical. Here, we demonstrate that Ru nanoparticles (NPs) enable NRR in 0.01 m HCl aqueous solution at very high energy efficiency, that is, very low overpotentials. Remarkably, the NRR occurs at a potential close to or even above the H+/H2 reversible potential, significantly enhancing the NRR selectivity versus the production of H2. NH3 yield rates as high as ≈5.5 mg h−1 m−2 at 20 °C and 21.4 mg h−1 m−2 at 60 °C were achieved at a redox potential (E) of −100 mV versus the reversible hydrogen electrode (RHE), whereas a highest Faradaic efficiency (FE) of ≈5.4 % is achievable at E=+10 mV vs. RHE. This work demonstrates the potential use of Ru NPs as an efficient catalyst for NRR at ambient conditions. This ability to catalyze NRR at potentials near or above RHE is imperative in improving the NRR selectivity towards a practical process as well as rendering the H2 viable as byproduct. Density functional theory calculations of the mechanism suggest that the efficient NRR process occurring on these predominantly Ru (0 0 1) surfaces is catalyzed by a dissociative mechanism.