Mossbauerite, a trivalent iron-only layered oxyhydroxide, has been recently identified as an electrocatalyst for water oxidation. We investigated the material as potential cocatalyst for photoelectrochemical water oxidation on semiconductor photoanodes. The band edge positions of mossbauerite were determined for the first time with a combination of Mott-Schottky analysis and UV-vis diffuse reflectance spectroscopy. The positive value of the Mott-Schottky slope and the flatband potential of 0.34 V vs reversible hydrogen electrode (RHE) identifies the material as an n-type semiconductor, but bare mossbauerite does not produce noticeable photocurrent during water oxidation. Type-II heterojunction formation by facile drop-casting with WO3 thin films yielded photoanodes with amended charge carrier separation and photocurrents up to 1.22 mA cm(-2) at 1.23 V vs RHE. Mossbauerite is capable of increasing the charge carrier separation at lower potential and improving the photocurrent during photoelectrochemical water oxidation. The rise in photocurrent of the mossbauerite-functionalized WO3 photoanode thus originates from improved charge carrier separation and augmented hole collection efficiency. Our results highlight the potential of mossbauerite as a second-phase catalyst for semiconductor electrodes.