The harvesting of sunlight by a photoelectrochemical (PEC) cell to split water into hydrogen and oxygen is an attractive strategy to store solar energy in the form of chemical bonds. The oxygen evolution reaction (OER) remains a bottleneck for the development of efficient PEC devices. Here we report a photoelectrochemical method to homogeneously deposit a cobalt-iron oxide (CoFeOx) catalyst on a nanostructured hematite photoanode. An ultrathin catalyst layer (<1 nm) yielded a 200 mV cathodic shift of onset potential and a photocurrent density of 1.6 and 2.5 mA cm(-2) at 1.0 V and 1.23 vs. RHE in 1 M KOH, respectively. We investigated the enhancement of photoactivity induced by the addition of the CoFeOx layer by impedance spectroscopy, photoluminescence, and by using H2O2 as a hole scavenger. This work points to the effective utilization of subnanometric coatings as efficient catalyst overlayers to enhance the OER activity of photoanodes.