Tandem polymer solar cells are used for light-driven electrochemical water splitting. To attain a high enough electrochemical potential a new wide band gap electron donor polymer (PTPTIBDT-OD) is developed and used in combination with PCBM as an electron acceptor in a tandem device architecture with two identical photoactive layers. This homo-tandem device comprises an intermediate ZnO/PEDOT:PSS/MoO3 charge recombination layer to connect the two subcells electrically and optically. The homo-tandem solar cell has an open-circuit voltage of 1.74 V and reaches a power conversion efficiency (PCE) of 5.3%. In combination with RuO2 as the electrocatalyst for oxygen evolution and RuO2 or Pt catalysts for hydrogen evolution, sunlight-driven electrochemical water splitting occurs with a solar-to-hydrogen conversion efficiency of ηSTH = 4.3%. Owing to the very high fill factor of the polymer tandem cell (0.73), water splitting takes place near the maximum power point of the homo-tandem solar cell. As a consequence, the difference between PCE and ηSTH is only due to the overpotential losses.