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The concept of hybrid tandem device structures that combine metal oxides with thin‐film semiconducting photoabsorbers holds great promise for large‐scale, robust, and cost‐effective bias‐free photoelectrochemical water splitting (PEC‐WS). This work highlights important steps toward the efficient coupling of high‐performance hematite photoanodes with multijunction thin‐film silicon photocathodes providing high bias‐free photocurrent density. The hybrid PEC‐WS device is optimized by testing three types of multijunction silicon photocathodes with the hematite photoanode: amorphous silicon (a‐Si:H) tandem: a‐Si:H/a‐Si:H and triple junction with microcrystalline silicon (μc‐Si:H): a‐Si:H/a‐Si:H/μc‐Si:H and a‐Si:H/μc‐Si:H/μc‐Si:H. The results provide evidence that the multijunction structures offer high flexibility for hybrid tandem devices with regard to tunable photovoltages and spectral matching. Furthermore, both photoanode and photocathode are tested under various electrolyte and light concentration conditions, respectively, with respect to their photoelectrochemical performance and stability. A 27 % enhancement in the solar‐to‐hydrogen conversion efficiency is observed upon concentrating light from 100 to 300 mW cm−2. Ultimately, bias‐free water splitting is demonstrated, with a photocurrent density of 4.6 mA cm−2 (under concentrated illumination) paired with excellent operation stability for more than 24 h of the all‐earth‐abundant and low‐cost hematite/silicon tandem PEC‐WS device., The authors acknowledge funding from Generalitat de Catalunya through the CERCA program, 2017 SGR 1246, 2017 SGR 327 and the Spanish MINECO projects MAT2014‐59961, ENE2016‐80788‐C5‐5‐R and ENE2017‐85087, together with the support from REPSOL, S. A. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV‐2017‐0706). IREC also acknowledges additional support from the European Regional Development Funds (ERDF, FEDER), (S)TEM part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program and the rest in the Nanoscience program of the University of Barcelona. The authors thank S. Moll (IEK‐5), M. Biset‐Peiró (IREC), and H. Xie (IREC) for their contribution to this work. F.U. acknowledges financial support from MINECO through Juan de la Cierva fellowship (FJCI‐2016–29147).V.S., K.W., and F.F. (authors from IEK‐5) thank the Deutsche Forschungsgemeinschaft (DFG) (Priority Program SPP 1613).