Molybdenum sulfide serves as an effective nonprecious metal catalyst for hydrogen evolution, primarily active at edge sites with unsaturated molybdenum sites or terminal disulfides. To improve the activity at a low loading density, two molybdenum sulfide clusters, [Mo 3 S 4 ] 4+ and [Mo 3 S 13 ] 2− , were investigated. The Mo 3 S x molecular catalysts were heterogenized on Sb 2 Se 3 with a simple soaking treatment, resulting in a thin catalyst layer of only a few nanometers that gave up to 20 mA cm −2 under one sun illumination. Both [Mo 3 S 4 ] 4+ and [Mo 3 S 13 ] 2− exhibit catalytic activities on Sb 2 Se 3 , with [Mo 3 S 13 ] 2− emerging as the superior catalyst, demonstrating enhanced photovoltage and an average faradaic efficiency of 100% for hydrogen evolution. This superiority is attributed to the effective loading and higher catalytic activity of [Mo 3 S 13 ] 2− on the Sb 2 Se 3 surface, validated by X-ray photoelectron and Raman spectroscopy.