Photoelectrochemical water-splitting devices, which use solar energy to convert water into hydrogen and oxygen, have been investigated for decades. Multijunction designs are most efficient, as they can absorb enough solar energy and provide sufficient free energy for water cleavage. However, a balance exists between device complexity, cost and efficiency. Water splitters fabricated using triple-junction amorphous silicon(1,2) or III-V-3 semiconductors have demonstrated reasonable efficiencies, but at high cost and high device complexity. Simpler approaches using oxide-based semiconductors in a dual-absorber tandem approach(4,5) have reported solar-to-hydrogen (STH) conversion efficiencies only up to 0.3% (ref. 4). Here, we present a device based on an oxide photoanode and a dye-sensitized solar cell, which performs unassisted water splitting with an efficiency of up to 3.1% STH. The design relies on carefully selected redox mediators for the dye-sensitized solar cell(6,7) and surface passivation techniques(8) and catalysts(9) for the oxide-based photoanodes.