A water-soluble perylene diimide (PDI), in the presence of triethanolamine (TEOA), is used as a metal-free photocatalyst for aqueous reversible addition–fragmentation chain transfer (RAFT) polymerization under green light. On the basis of cyclic voltammetry, fluorescence spectroscopy and polymerization results, a complex but interesting “hybrid” mechanism is proposed that involves photosensitized initiation of polymerization and photoinduced electron transfer RAFT (PET-RAFT) polymerization. Pseudo-first order polymerization kinetics is observed after an initial inhibition period and the polymerization rate scales with the concentrations of TEOA and PDI. Addition of cucurbit[7]uril (CB[7]) enhances the polymerization rate due to the formation of PDI/CB[7] complexes that reduce the aggregation of PDI as well as lower the frontier orbital energy levels. The photopolymerization can be activated and deactivated by switching the light on and off. Controlled photopolymerizations of N,N-dimethylacrylamide up to an ultrahigh molecular weight (∼10⁶ g mol⁻¹) can be efficiently conducted using an extremely low concentration (1 ppm relative to monomer) of the supramolecular photocatalyst. The synthesis of well-defined block copolymers from different monomer families is also demonstrated.