Efficient TaON and Ta3N5 photoanodes for water splitting were fabricated on conducting glass support (FTO). A necking treatment, which forms effective contacts between TaON (or Ta3N5) particles, afforded a significant increase in the photocurrent. Furthermore, loading of IrO2·nH2O nanoparticles as a cocatalyst for water oxidation improved the photocurrent of the TaON (or Ta3N5) photoanode. The incident photon to charge carrier efficiencies (IPCEs) of the TaON and Ta3N5 photoanodes were calculated to be ca. 76% at 400 nm and ca. 31% at 500 nm, respectively, at 1.15 V vs. reversible hydrogen electrode (RHE) in aqueous Na2SO4 solution. Overall water splitting into H2 and O2 under visible light was demonstrated using an IrO2·nH2O-loaded TaON (or Ta3N5) photoanode combined with a Pt electrode under an externally applied bias (TaON: > 0.6 V, Ta 3 N 5 : > 1.0 V).