Electrocatalytic nitrate reduction reaction (NO₃RR) offers a unique rationale for green NH₃ synthesis, yet the lack of high-efficiency NO₃RR catalysts remains a great challenge. In this work, we show that Au nanoclusters anchored on TiO₂ nanosheets can efficiently catalyze the conversion of NO₃RR-to-NH₃ under ambient conditions, achieving a maximal Faradic efficiency of 91%, a peak yield rate of 1923 μg·h⁻¹·mg_cat.⁻¹, and high durability over 10 consecutive cycles, all of which are comparable to the recently reported metrics (including transition metal and noble metal-based catalysts) and exceed those of pristine TiO₂. Moreover, a galvanic Zn-nitrate battery using the catalyst as the cathode was proposed, which shows a power density of 3.62 mW·cm⁻² and a yield rate of 452 μg·h⁻¹·mg_cat.⁻¹. Theoretical simulations further indicate that the atomically dispersed Au clusters can promote the adsorption and activation of NO₃⁻ species, and reduce the NO₃RR-to-NH₃ barrier, thus leading to an accelerated cathodic reaction. This work highlights the importance of metal clusters for the NH₃ electrosynthesis and nitrate removal.