Ta₃N₅ is regarded as a promising photocatalyst for solar water splitting because of its excellent visible light absorption characteristics and simple composition. Conventional Ta₃N₅ photocatalysts prepared from oxide precursors typically comprise aggregated polycrystalline particles with defects and grain boundaries that reduce the water oxidation activity of the material. In the present work, well-dispersed Ta₃N₅ nanoparticulate single crystals were synthesized via a mild nitridation process using pure Ta metal nanopowder or Ta nanopowder mixed with NaCl. The resulting high-quality Ta₃N₅ nanoparticles, after loading with an oxygen evolution cocatalyst, exhibited impressively high photocatalytic performance during O₂ evolution from a sacrificial AgNO₃ solution, with an apparent quantum yield of 9.4% at 420 nm. Our findings suggest a new approach to the facile fabrication of nanostructured single-crystal photocatalysts for efficient solar water splitting, based on the use of metal nanopowders.