The production of fuels from sunlight represents one of the main challenges in the development of a sustainable energy system(1-5). Hydrogen is the simplest fuel to produce and although platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution(6-9), earth-abundant alternatives are needed for large-scale use(10-15). We show that bioinspired molecular clusters based on molybdenum and sulphur evolve hydrogen at rates comparable to that of platinum(6). The incomplete cubane-like clusters (Mo3S4) efficiently catalyse the evolution of hydrogen when coupled to a p-type Si semiconductor that harvests red photons in the solar spectrum. The current densities at the reversible potential match the requirement of a photoelectrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10% (ref. 16). The experimental observations are supported by density functional theory calculations of the Mo3S4 clusters adsorbed on the hydrogen-terminated Si(100) surface, providing insights into the nature of the active site.