Clinical　applications　of　current　photodynamic　therapy　(PDT)　agents　are　often　limited　by　their　low　singlet　oxygen　(O-1(2))　quantum　yields,　as　well　as　by　photobleaching　and　poor　biocompatibility.　Here　we　present　a　new　PDT　agent　based　on　graphene　quantum　dots　(GQDs)　that　can　produce　O-1(2)　via　a　multistate　sensitization　process,　resulting　in　a　quantum　yield　of　similar　to　1.3,　the　highest　reported　for　PDT　agents.　The　GQDs　also　exhibit　a　broad　absorption　band　spanning　the　UV　region　and　the　entire　visible　region　and　a　strong　deep-red　emission.　Through　in　vitro　and　in　vivo　studies,　we　demonstrate　that　GQDs　can　be　used　as　PDT　agents,　simultaneously　allowing　imaging　and　providing　a　highly　efficient　cancer　therapy.　The　present　work　may　lead　to　a　new　generation　of　carbon-based　nanomaterial　PDT　agents　with　overall　performance　superior　to　conventional　agents　in　terms　of　O-1(2)　quantum　yield,　water　dispersibility,　photo-and　pH-stability,　and　biocompatibility.