This study presents a novel photodynamic therapy (PDT) agent based on graphene quantum dots (GQDs) that can produce singlet oxygen (1O2) via a multistate sensitization process, achieving a quantum yield of approximately 1.3, the highest reported for PDT agents. GQDs exhibit broad absorption in the UV and visible regions and strong deep-red emission. In vitro and in vivo studies demonstrate that GQDs can be used as effective PDT agents, providing both imaging capabilities and highly efficient cancer therapy. The high 1O2 quantum yield, water dispersibility, photostability, pH stability, and biocompatibility of GQDs make them a promising candidate for next-generation carbon-based nanomaterials in PDT.This study presents a novel photodynamic therapy (PDT) agent based on graphene quantum dots (GQDs) that can produce singlet oxygen (1O2) via a multistate sensitization process, achieving a quantum yield of approximately 1.3, the highest reported for PDT agents. GQDs exhibit broad absorption in the UV and visible regions and strong deep-red emission. In vitro and in vivo studies demonstrate that GQDs can be used as effective PDT agents, providing both imaging capabilities and highly efficient cancer therapy. The high 1O2 quantum yield, water dispersibility, photostability, pH stability, and biocompatibility of GQDs make them a promising candidate for next-generation carbon-based nanomaterials in PDT.