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Photosensitizers (PS) are ideally devoid of any activity in the absence of photoactivation, and rely on molecular oxygen for the formation of singlet oxygen ((1)O2) to produce cellular damage. Off-targets and tumor hypoxia therefore represent obstacles for the use of PS for cancer photodynamic therapy. Herein, we describe the characterization of OR141, a benzophenazine compound identified through a phenotypic screening for its capacity to be strictly activated by light and to kill a large variety of tumor cells under both normoxia and hypoxia. This new class of PS unraveled an unsuspected common mechanism of action for PS that involves the combined inhibition of the mammalian target of rapamycin (mTOR) signaling pathway and proteasomal deubiquitinases (DUBs) USP14 and UCH37. Oxidation of mTOR and other endoplasmic reticulum (ER)-associated proteins drives the early formation of high molecular weight (MW) complexes of multimeric proteins, the concomitant blockade of DUBs preventing their degradation and precipitating cell death. Furthermore, we validated the antitumor effects of OR141 in vivo and documented its highly selective accumulation in the ER, further increasing the ER stress resulting from (1)O2 generation upon light activation.