Photosynthetic hydrogen (photoH2) production is an elegant approach to storing solar energy. The most efficient strategy is to couple the hydrogen‐producing enzyme, the hydrogenase (H2ase), directly to photosystem I (PSI), which is a light‐driven nanomachine found in photosynthetic organisms. PSI–H2ase fusions have been tested in vivo and in vitro. Both approaches have each their specific advantages and drawbacks. Here, a system to combine both approaches by assembling PSI–H2ase fusions in vivo for in vitro photoH2 production is established. For this, cyanobacterial PSI–H2ase fusion mutants are generated and characterized concerning photoH2 production in vivo. The chimeric protein is purified and embedded in a redox polymer on an electrode where it successfully produces photoH2 in vitro. The combination of in vivo and in vitro processes comes along with reciprocal benefits. The in vivo assembly ensures that the chimeric protein is fully functional and suited for the fabrication of bioelectrodes in vitro. At the same time, the photoelectrochemical in vitro characterization now permits to analyze the assemblies in detail. This will open avenues to optimize in vivo and in vitro approaches for photoH2 production in a target‐oriented manner in the future. Cyanobacterial mutants that express photosystem I and hydrogenase assembled in vivo are created. The chimeric fusion protein is purified and embedded in a redox polymer over an electrode, which is used for successful photoH2 production in vitro. Photoelectrochemical characterizations now permit a detailed analysis of the assemblies enabling optimization of in vivo and in vitro approaches for photoH2 production.