Hydrogen sulfide monitoring has become essential in the natural gas industry, biogas production, wastewater treatment plants, paper mills, sewers, and landfills of waste due to its toxic, irritating, extremely flammable, and corrosive features. However, each of the current monitoring technologies (gas chromatography, lead acetate tape, electrochemical, UV and NIR absorption) has its own limitations. Furthermore, the existing luminescent molecular probes for HS cannot monitor it continuously due to the irreversibility of their reaction with the analyte. Herein, we report the development and application of the first reversible HS luminescent sensor. The sensing layer capitalizes on the highly photooxidizing phosphorescent [bis(1,10-phenanthroline)(1,4,5,8-tetraazaphenanthrene)]ruthenium(II) dication immobilized on alkali-treated silica microspheres, interrogated with a dedicated fiber-optic phase-sensitive luminometer. The chemosensing mechanism is a fully reversible electron transfer from the analyte to the photoexcited dye. The HS optosensor exhibits a 0.34-50 ppmv dynamic range, a limit of detection equal to 0.025 ppmv, repeatability, and reproducibility better than 3.2%, plus response and recovery times ( t and t) shorter than 240 s. The HS luminescent sensor performance has been verified for more than six months in a biomethane production plant, showing an excellent stability with automatic daily maintenance.