Accurate　detection　of　hydrogen　sulfide　(H₂S)　in　live　cells　is　critical　for　understanding　its　role　in　disease　but　is　challenged　by　poor　biocompatibility,　signal　instability,　and　background　interference.　Here,　we　developed　a　novel　surface-enhanced　Raman　scattering　(SERS)　hydrogel　platform　based　on　carboxymethyl　cellulose‑silver　nanoparticles　embedded　in　agarose　(CMC-Ag　NPs-AG).　This　platform　is　prepared　via　simple　physical　crosslinking,　enabling　uniform　Ag　NP　dispersion　and　stable　encapsulation.The　hydrogel　shows　excellent　swelling　capacity,　antibacterial　activity　(＞99　%　inhibition),　and　high　biocompatibility　(＞98　%　cell　viability).　Leveraging　the　specific　formation　of　Ag₂S　upon　H₂S　exposure,　the　platform　enables　ultrasensitive　and　selective　detection.　It　exhibits　a　wide　linear　range　(0.15–600　μM),　a　low　detection　limit　of　0.15　μM,　and　excellent　reproducibility　(RSD　=　1.10　%).　It　also　demonstrates　strong　anti-interference　performance　against　structurally　similar　sulfur-containing　molecules.Furthermore,　this　platform　was　successfully　applied　to　monitor　endogenous　H₂S　in　A549　cancer　cells　under　varying　culture　conditions.　The　results　confirm　its　potential　for　real-time,　in　situ　detection　of　gaseous　signaling　molecules　in　complex　biological　environments.　This　work　offers　a　robust　and　biocompatible　SERS　strategy　for　H₂S　analysis　and　provides　new　insights　into　tumor　metabolism　and　disease　diagnosis.　©　2025　Elsevier　B.V.