Tandem solar cells using different bandgap absorbers allow efficient photovoltaic conversion in a wide range of the solar spectrum. The optical gaps of the dye-sensitized solar cell and the Cu(In,Ga)Se2 solar cell are ideal for application in double-junction devices and a mechanically stacked device has been reported recently. We report on the monolithic integration of these subcells to cut optical losses at needless interfaces and material costs, achieving 12.2% conversion efficiency at full sunlight. The high open-circuit voltage confirms the series connection, but corrosion of the Cu(In,Ga)Se2 cell by the redox mediator (I−/I3− couple) of the dye-sensitized cell and an associated voltage loss (~140 mV) limits performance.