A two-step photocatalytic water splitting (Z-scheme) system consisting of a modified ZrO2/TaON species (H2 evolution photocatalyst), an O2 evolution photocatalyst, and a reversible donor/acceptor pair (i.e., redox mediator) was investigated. Among the O2 evolution photocatalysts and redox mediators examined, Pt-loaded WO3 (Pt/WO3) and the IO3-/I- pair were respectively found to be the most active components. Combining these two components with Pt-loaded ZrO2/TaON achieved stoichiometric water splitting into H2 and O2 under visible light, achieving an apparent quantum yield of 6.3% under irradiation by 420.5 nm monochromatic light under optimal conditions, 6 times greater than the yield achieved using a TaON analogue. To the best of our knowledge, this is the highest reported value to date for a nonsacrificial visible-light-driven water splitting system. The high activity of this system is due to the efficient reaction of electron donors (I- ions) and acceptors (IO3- ions) on the Pt/ZrO2/TaON and Pt/WO3 photocatalysts, respectively, which suppresses undesirable reverse reactions involving the redox couple that would otherwise occur on the photocatalysts. Photoluminescence and photoelectrochemical measurements indicated that the high activity of this Z-scheme system results from the moderated n-type semiconducting character of ZrO2/TaON, which results in a lower probability of undesirable electron-hole recombination in ZrO2/TaON than in TaON.