In this work, we adopted a fully computer-guided strategy in discovering an efficient pH-switchable organic photocatalyst (OPC), unprecedentedly turning colorless at pH 5 and recovering strong visible-light absorption and photoactivity at pH 7. This is the first example of an OPC design fully guided by comprehensive density functional theory (DFT) studies covering electrostatic, electrochemical, and photophysical predictions. Characterization of the designed OPC after synthesis confirmed the computational predictions. We applied this OPC to mediate an aqueous photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization under green LED light (nominal emission wavelength: 530 nm, 5 mW/cm2). We demonstrated that the polymerization can be reversibly ceased by a slight change of pH (pH ≤ 5.0) or in the absence of light. Furthermore, we demonstrated that the polymerization rate could be significantly retarded by bubbling carbon dioxide into the reaction solution under visible light. Conversely, the rate could be fully recovered via exposure to nitrogen gas. This is the first example of a pH and light dual-gated polymerization system with complete and reversible inhibition.