Green H₂ has been recognized as an important element in efforts to decarbonize our fossil fuel-dependent society. One approach to produce green H₂ is solar water splitting in a photoelectrochemical (PEC) device. Solar-to-hydrogen (STH) efficiencies of up to 30% have been demonstrated[1] but studies have shown that this approach still results in a relatively high levelized cost of hydrogen (LCOH) of ~10 US$ per kg of H₂.[2-3] This is ca. one order of magnitude higher than that of hydrogen produced via steam methane reforming (SMR), which forms the bulk of the currently produced H₂. A possible solution is to incorporate an upgrading process of biomass feedstock that generates valuable chemicals into the solar water splitting device. This is expected to not only decrease the overall LCOH but also introduce an alternative renewable pathway in chemical manufacturing. In this study, we propose the concept of solar-driven hydrogenation of biomass-derived feedstock. Photoelectrochemically generated H₂ in our solar water splitting device is coupled in situ with the homogenously catalyzed hydrogenation of itaconic acid (IA) to methyl succinic acid (MSA). IA has been identified by the US Department of Energy as one of the twelve building blocks that possess the potential to be transformed subsequently into several high-value bio-based chemicals or materials. MSA is a valuable chemical compound with an estimated global market size of up to ~15,000 tonnes, whose derivatives are ubiquitously used as solvents in cosmetics, polymer synthesis, binders in powder coatings, and organic synthesis, especially for pharmaceutical synthesis.[4-5] Our coupled hydrogenation approach—performed in the PV-electrolyzer and PEC configurations using III-V PV cells and BiVO₄-based photoelectrode, respectively—successfully demonstrate solar-driven H₂-to-MSA conversion as high as 60%. In comparison to the non-coupled approach (i.e., direct hydrogenation), our coupled system offers synergistic benefits in terms of prolonged durability and a higher degree of flexibility toward other important chemical transformation reactions. In addition, life-cycle net energy assessment and technoeconomic analysis results show that adding the coupled hydrogenation process significantly lowers the energy payback time and the LCOH, respectively, to a point that is competitive even with SMR-produced H₂.[6] Further implications and optimization potentials of the coupled PEC hydrogenation approach, also beyond the demonstrated hydrogenation of IA to MSA, will be discussed.