Hydrogen sulfide (H₂S) poses a significant threat to human health even at trace levels. Rapid and reliable H₂S detection with high sensitivity is of paramount importance. Herein, copper oxide and tin dioxide comodified reduced graphene oxide (CuO/SnO₂/RGO) composite nanosheets (CSRs) were synthesized via a one-step microwave-assisted method and used as functional sensing layers for H₂S detection. The composite consists of secondary CuO and SnO₂ nanoparticles, grown in situ and uniformly anchored onto the reduced graphene oxide (RGO) surfaces. The 3-CSR composite with a Cu : Sn molar ratio of 3 : 7 has a particle size range from 6.3 to 25.1 nm with an average diameter of 11.8 nm. The formation mechanism stems from the initial coordination of Sn²⁺ and Cu²⁺ ions with the oxygen-containing functional groups on the graphene oxide (GO) surface. The CSR sensor demonstrates an exceptional sensing response to H₂S, which is significantly modulated by the Cu/Sn molar ratio. The optimized 3-CSR composite exhibits superior gas-sensing properties, achieving a remarkable response of 28,233 toward 50 ppm H₂S with an ultrafast response time of merely 2 s at a low operating temperature of 80 °C. The enhanced performance is attributed to the synergistic effects of numerous p–n heterojunctions, together with the high RGO network, high specific surface area, and oxygen vacancy defects, which promote gas adsorption and charge transfer.