PGR5 is involved in cyclic electron flow around Photosystem I (PSI) and is essential for photoprotection in Arabidopsis. The study identifies PGR5 as a novel thylakoid membrane protein that facilitates electron transfer from ferredoxin to plastoquinone, contributing to the generation of a proton gradient (ΔpH) that induces thermal dissipation under conditions of reduced Calvin cycle activity. This pathway also limits the overreduction of the acceptor side of PSI, preventing photoinhibition. The pgr5 mutant exhibits high chlorophyll fluorescence under high light conditions, indicating impaired thermal dissipation. PGR5 functions in the photoprotection of PSI under acceptor side limitation by draining electrons trapped in PSI. The PGR5 gene encodes a thylakoid membrane protein that is essential for cyclic electron flow around PSI and is involved in the acidification of the thylakoid lumen, which is crucial for photoprotection. The PGR5 pathway is essential for inducing thermal dissipation and preventing PSI photoinhibition. The study demonstrates that PGR5 is involved in an alternative electron transport pathway that may be redundant with the NDH complex. The PGR5-dependent pathway is crucial for maintaining ΔpH and inducing thermal dissipation under stress conditions. PGR5 is essential for the photoprotection of PSI by preventing overreduction of the acceptor side and limiting electron transport under conditions of limited electron acceptors. The PGR5 pathway is essential for the photoprotection of PSI and plays a critical role in the induction of thermal dissipation under high light conditions.PGR5 is involved in cyclic electron flow around Photosystem I (PSI) and is essential for photoprotection in Arabidopsis. The study identifies PGR5 as a novel thylakoid membrane protein that facilitates electron transfer from ferredoxin to plastoquinone, contributing to the generation of a proton gradient (ΔpH) that induces thermal dissipation under conditions of reduced Calvin cycle activity. This pathway also limits the overreduction of the acceptor side of PSI, preventing photoinhibition. The pgr5 mutant exhibits high chlorophyll fluorescence under high light conditions, indicating impaired thermal dissipation. PGR5 functions in the photoprotection of PSI under acceptor side limitation by draining electrons trapped in PSI. The PGR5 gene encodes a thylakoid membrane protein that is essential for cyclic electron flow around PSI and is involved in the acidification of the thylakoid lumen, which is crucial for photoprotection. The PGR5 pathway is essential for inducing thermal dissipation and preventing PSI photoinhibition. The study demonstrates that PGR5 is involved in an alternative electron transport pathway that may be redundant with the NDH complex. The PGR5-dependent pathway is crucial for maintaining ΔpH and inducing thermal dissipation under stress conditions. PGR5 is essential for the photoprotection of PSI by preventing overreduction of the acceptor side and limiting electron transport under conditions of limited electron acceptors. The PGR5 pathway is essential for the photoprotection of PSI and plays a critical role in the induction of thermal dissipation under high light conditions.