Successful applications of photodynamic therapy (PDT) in cancer treatment require the development of effective photosensitizers with controllable singlet oxygen generation. Here we report a ubiquinone-BODIPY photosensitizer that self-assembles into nanoparticles (PS-Q-NPs) and undergoes selective activation and deaggregation within the highly reductive intracellular environment of tumor cells. PS-Q-NPs are highly stable in aqueous buffer solution, and exhibit minimal fluorescence and photosensitization due to a rapid non-radiative relaxation process. Upon endocytosis by cancer cells, reduction of the ubiquinone moiety by intracellular glutathione (GSH) triggers the conversion of the aggregated hydrophobic precursor into the active hydrophilic carboxylate derivative PS-A. The conversion results in enhanced fluorescence and therapeutic singlet oxygen generation, portending to its application as an activatable photosensitizer for fluorescence imaging-guided photodynamic cancer therapy.