The onset potential of photoelectrochemical water oxidation on ultrathin hematite was improved by up to 200 mV by the chemical bath deposition of 13-group oxides as overlayers. It is proposed that the corundum-type overlayers released lattice strain of the ultrathin hematite layer and decreased the density of surface states. Particularly, a Ga2O3 overlayer exhibited an enhanced photocurrent attributed to stoichiometric water splitting near the onset potential. The photocurrent was sustained over a day, attesting to its outstanding performance and durability for water splitting.