A dual-responsive injectable thermogel based on decanoyl glycol chitosan (DGC) was developed for controlled drug delivery applications. DGC was synthesized by N-decanoylation of glycol chitosan and exhibited a unique temperature- and pH-dependent sol-gel-sol transition. Dynamic light scattering confirmed the formation of nanoscale assemblies whose size varied in response to environmental conditions. The thermogel efficiently encapsulated doxorubicin (DOX), enabling sustained and pH-dependent drug release. In vivo experiments demonstrated in situ gelation with excellent biocompatibility and negligible inflammatory or cytotoxic effects. These results highlight the potential of DGC as a smart injectable biomaterial for precision drug delivery and tissue engineering.