The reaction of water splitting has been investigated widely in literature motivated by the possibility of producing molecular hydrogen (H₂) as a clean, renewable, and abundant source of energy. Photocatalysis is a promising strategy to generate hydrogen from renewable sources, such as water and sunlight. However, the efficiencies of contemporary photocatalytic systems are not yet enough to contribute substantially to the world's energy demand. One important issue that requires urgent attention is the design of earth-abundant, tunable, and selective co-catalysts. Among various candidates, transition-metal-based sulfides – such as those from the MoS_x-family – have shown excellent co-catalytic properties due to the presence of suitable active sites for electrochemical H₂ production. Thiomolybdates such as [Mo₃S₁₃]^2-are molecular clusters that feature well-defined structures, compositions, and geometries, which may allow for in-depth studies and understanding of the active sites, reaction mechanisms, and dynamic nature of the catalytic processes. Several research groups provided insights into the thiomolybdates heterogenization onto different supports, catalytic active sites, hydrogen evolution reaction (HER) mechanisms, and activity. Therefore, we designed a model photocatalyst by immobilizing the noble-metal- and carbon-free thiomolybdate [Mo₃S₁₃]^2- clusters – as molecular co-catalysts – onto photoactive metal oxide supports via covalent linkage for photocatalytic water splitting. The [Mo₃S₁₃]^2- clusters heterogenized onto TiO₂are shown to be highly active and stable co-catalysts for hydrogen evolution reaction (HER). Since the absorption of TiO₂ is limited to the UV region of the solar spectrum, it consequently led us to explore other supports capable of absorbing visible light. Therefore, these [Mo₃S₁₃]^2- cocatalysts were immobilized onto visible light-absorbing graphitic carbon nitride support to examine their HER performance and to draw a direct comparison of homogeneous and heterogeneous photocatalysis under similar reaction conditions. The heterogenized clusters were shown to be highly stable for the long-term, however, the stability of these clusters was compromised over activity under homogeneous HER conditions. These studies provide a benchmark for the successful heterogenization of an inorganic molecular cluster as a co-catalyst for light-driven HER and give the incentive to explore other oxothiometelates.