Barium tantalum oxynitride (BaTaO2N) has been demonstrated to be an attractive catalyst for photo-electrochemical water splitting, especially oxygen evolution reactions. However, the hydrogen production rates and charge transfer resistances of BTN leave much to be desired from a practical standpoint. In fact, it has never been reported for photocathode applications. On the other hand, CuO has been reported for hydrogen evolution reactions. Given this, here nanocomposites of BaTaO2N and copper oxide quantum dots (CuO QDs), where the latter are digestively ripened, are reported for photoelectrochemical water splitting on a carbon paper substrate. To assess the role of ultra-small quantum dots, composites of commercially available CuO nanoparticles with BaTaO2N were also tested. BaTaO2N and CuO QD nanocomposites performed well and showed a high photocurrent density of 3.1 mA cm−2 at 0 V vs RHE. This was also accompanied by reduced charge transfer resistance (225.4 Ω) due to the high interfacial surface area provided by nanoscopic CuO on BaTaO2N.