Direct solar-to-hydrogen conversion via water splitting was demonstrated in an integrated photovoltaic–electrochemical (PV–EC) device using a hydrogenated amorphous silicon thin film tandem junction (a-Si:H/a-Si:H) solar cell as photocathode. The solar cell was adapted to provide sufficient photovoltage to drive both the hydrogen and oxygen evolution reactions. The best results, in terms of photoelectrochemical stability and performance, were obtained with an Ag/Pt layer stack as H2 evolving photocathode back contact and with a RuO2 counter electrode for O2 evolution. Under irradiation by simulated sunlight (AM 1.5 spectrum with 100 mW/cm2), we achieved 6.8% solar-to-hydrogen efficiency at 0 V applied bias in a two-electrode set-up. This sets a fresh benchmark for integrated thin film silicon tandem based photoelectrochemical devices. In addition, the photovoltage at constant current (−3 mA/cm2) was measured over a prolonged period of time and revealed an excellent chemical stability (operation over 50 h) of the photocathode. Furthermore, we present an empirical serial circuit model of the PV–EC device, in which the corresponding photovoltaic and electrochemical components are decoupled. This allows for a detailed comparison between the solar cell and the PV–EC cell characteristics, from which the relevant loss processes in the overall system could be identified. The model was further used to compare calculated and measured photocurrent–voltage characteristics of the investigated PV–EC device which showed excellent agreement.