A LaNbON2 semiconductor is capable of absorbing long light wavelengths up to 750 nm, which indicates a promising n-type material for solar water splitting to produce hydrogen. In this study, we present enhanced photocatalytic and photoelectrochemical activities over the particulate LaNbON2 during visible-light-driven water oxidation. La-rich oxide mixtures, prepared by the calcination of La2O3–Nb2O5 with La/Nb ratios greater than 1.00, were proposed as a starting precursor for the synthesis of less-defective LaNbON2. The La-rich condition caused the synthesis of smaller LaNbON2 particles with larger BET surface areas, which were favorable for the complete nitridation under mild conditions. The LaNbON2 particles prepared from the La/Nb ratio of 1.20, which were capable of absorbing a wide range of visible light up to approximately 750 nm and above, completed the nitridation at 1123 K for 15 h. The La-rich condition promoted reduced bulk defect density of LaNbON2 and suppressed the formation of Nb3+ species at the oxynitride surface during nitridation. These differences led to water oxidation photocurrent over the particulate LaNbON2 photoanode under AM 1.5 G simulated sunlight, which has never previously been reported. Therefore, the synthesis strategy using La-rich oxide was effective at preparing less-defective LaNbON2, thus improving the photoactivity.