In this article, we have shown that visible-light exposure of graphitic carbon nitride, (g‐C3N4, a 2D semiconductor polymeric material) results in the formation of reactive oxygen species, causing death of cancer cells in vitro and in vivo, as evident by classical methods as well as omics approaches. We suggest that photo‐excitation of g‐C3N4 could be used effectively in a photodynamic therapy protocol for cancer therapy without using any other nanocarrier, additional photosensitizer or a chemotherapeutic drug.