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Photoacid generators (PAGs) have facilitated a number of technology breakthroughs in the electronic, coating, and additive manufacturing industries. Traditionally, PAGs that contain weakly coordinating anions, such as PF6-, generate Brønsted superacids under UV irradiation for rapid cationic polymerizations. However, PAGs with strongly coordinating anions remain under-utilized as they form weak acids that are inefficient or even incapable of initiating polymerization. To expand the scope of potential counteranions in PAGs, we leveraged a thiophosphoramide hydrogen bond donor (HBD) to catalyze photoinitiated cationic polymerizations from diphenyliodonium PAGs. Through the formation of hydrogen bonds between the HBD and PAG counteranion, acceleration of the polymerization rate was observed for a range of non-coordinating and coordinating anions. The effect of the HBD on the polymerization kinetics was investigated by 1H-NMR titrations and geometry optimizations. Extending HBD catalysis beyond photopolymerizations, addition of HBD also enabled hydrochloric acid to initiate controlled reversible addition-fragmentation chain transfer (RAFT) polymerization under ambient conditions. With the versatility of HBD, there is potential to access initiation systems that were previously believed to be impractical for cationic polymerization.