Here a novel tris(keto-hydrazone) monomer having secondary amines and alkoxy groups to detect toxic hydrogen sulfide (H2S) gas is reported. The as-synthesized tris(keto-hydrazone) monomer is successfully integrated on a micro-fabricated device to realize an organic capacitive sensor. The organic sensor's quantitative detection capability toward H2S gas and its specificity against the other toxic gases and volatile organic compounds are investigated. The capacitance sensor achieves an excellent sensitivity (80% parts per million–1) toward H2S gas with an experimental limit of detection of around 25 parts per billion. Besides, the fabricated capacitive sensor displays minimal response to humidity (0.005% RH–1), and high ambient stability (≈8 months) without compromising sensing performance. Furthermore, the energy-dispersive X-ray spectroscopy spectrum analysis confirms the adsorption of sulfur atoms over the surface of the monomer after the exposure to H2S gas. After that, a short purge of N2 gas would suffice to revive the whole device and can work with negligible losses.