Strontium niobium oxynitride (SrNbO2N) particles were coated on fluorine-doped tin oxide (FTO) glass and examined as a photoelectrode for water splitting under visible light in a neutral aqueous solution (Na 2SO4, pH ≈ 6). SrNbO2N, which has a band gap of ca. 1.8 eV, acted as an n-type semiconductor and generated an anodic photocurrent assignable to water oxidation upon irradiation with visible-light photons with wavelengths of up to 700 nm, even without an externally applied potential. Under visible light (λ > 420 nm) with an applied potential of +1.0-1.55 V vs RHE, nearly stoichiometric H2 and O2 evolution was achieved using a SrNbO2N/FTO electrode modified with colloidal iridium oxide (IrO2) as a water oxidation promoter. This study presents the first example of photoelectrochemical water splitting involving an n-type semiconductor with a band gap smaller than 2.0 eV that does not require an externally applied potential.