Graphitic-phase carbon nitride (g-CN) nanosheets, the newly emerging two-dimensional (2D) layered nanomaterials, have been demonstrated to be promising bioimaging agents due to their high photoluminescence (PL) quantum yields, good biocompatibility and low toxicity. However, the therapeutic applications of g-CN nanosheets have not been explored until now. In this study, we have proven for the first time that g-CN nanosheets can be used as efficient photosensitizers for photodynamic tumor therapy and as pH-responsive nanocarriers for drug delivery. On one hand, as photosensitizers, g-CN nanosheets are able to generate reactive oxygen species (ROS) and kill cancer cells efficiently under low-intensity light irradiation (20 mW cm). On the other hand, as nanocarriers, g-CN nanosheets possess an ultrahigh drug-loading capacity owing to their high surface-to-volume ratio. More importantly, g-CN nanosheets loaded with the anticancer drug doxorubicin (DOX) exhibit a pH-responsive release property which is beneficial for the delivery of DOX into cancer cells for chemotherapy. Furthermore, due to their high PL quantum yields, the fluorescent g-CN nanosheets can enable visualization of the delivery. These findings demonstrated the potential of g-CN nanosheets as low-toxic and biocompatible photosensitizers and pH-responsive drug nanocarriers for biomedical applications.