Here we report the highly effective detection of hydrogen sulfide (H₂S) gas by redox reactions based on single-walled carbon nanotubes (SWCNTs) functionalized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a catalyst and we also discuss the important role of water vapor in the electrical conductivity of SWCNTs during the sensing of H₂S molecules. To explore the H₂S sensing mechanism, we investigate the adsorption properties of H₂S on carbon nanotubes (CNTs) and the effects of the TEMPO functionalization using first-principles density functional theory (DFT) and we summarize currentchanges of devices resulting from the redox reactions in the presence of H₂S. The semiconducting-SWCNT (s-SWCNT) device functionalized with TEMPO shows a very high sensitivity of 420% at 60% humidity, which is 17 times higher than a bare s-SWCNT device under dry conditions. Our results offer promising prospects for personal safety and real-time monitoring of H₂S gases with the highest sensitivity and low power consumption and potentially at a low cost.