We developed a thermoswitchable polymeric photosensitizer (T-PPS) by conjugating PS (Pheophorbide-a, PPb-a) to a. temperature-responsive polymer backbone of biacoinpatible hydroxypropyl cellulose. Self-quenched PS molecules linked in close proximity by pi-pi stacking in T-PPS were easily transited to an active monomeric state by the temperature-induced phase transition of polymer backbones. The temperature: responsive intermolecular interaction changes of PS Molecules in T-PPS were demonstrated in synchrotron small-angle X-ray scattering and UV-vis spectrophotometer analysis. The T-PPS allowed switchable activation and synergistically enhanced cancer cell killing effect at the hyperthermia temperature (45 degrees C). Our developed T-PPS has the considerable potential not only as a new class of photomedicine in clinics but also as a biosensor based on temperature responsiveness.