The encapsulation of the complex [Ni(dto)(2)](2-) Within an oxothiododecamolybdic cyclic cluster has been investigated. The resulting molybdenum ring, [Mo12O12S12(OH)(12)(Ni(dto)(2))](2-), corresponds to the first example of the {Mo2O2S2}-based assembly arranged around a 3d transition-metal complex. It was unambiguously characterized in the solid state and in solution by FT-IR spectroscopy, single-crystal X-ray diffraction, NMR, UV-visible spectroscopy, and electrospray ionization-high-resolution mass spectrometry (ESI-HRMS). The latter technique revealed to be a powerful tool for the characterization of templated molybdenum ring systems in solution and gave excellent results in high resolution. The electronic spectrum of [Mo12O12S12(OH)(12)(Ni(dto)(2))](2-) evidenced a strong red shift of the LMCT bands attributed to the complex [Ni(dto)(2)](2-), suggesting significant variations of the electronic properties upon its encapsulation within the Mo-12 ring. These differences were demonstrated by electrochemical studies in CH3CN, which also revealed, for both compounds [Ni(dto)(2)](2-) and [Mo12O12S12(OH)(12)(Ni(dto)(2))](2-), electrocatalytic properties for the reduction of protons. These results evidence the ability of dithioxalato complexes to act as electrocatalysts for the hydrogen evolution reaction (HER) and confirm such a property for oxothiomolybdenum rings.