Regulation of proton motive force (pmf) via ATP synthase activity is a critical mechanism by which photosynthetic organisms maintain redox homeostasis and control the activation and inactivation of photoprotective responses under fluctuating light conditions. Here, we used time-resolved electrochromic shift measurements to investigate pmf dynamics in the C3 model plant Arabidopsis thaliana and the C4 model grass Setaria viridis. Our results reveal that ATP synthase is dynamically regulated during light fluctuations, but in Arabidopsis this regulation could not be explained by the established light-induced reduction of the CF₁γ subunit by thioredoxins, suggesting alternative control mechanisms. The PROTON GRADIENT REGULATION 5 (PGR5) protein, previously proposed to facilitate cyclic electron transport (CET) in plants and algae, also has a potential role in regulation of ATP synthase. We therefore investigated pmf dynamics, cytochrome f redox changes, and linear and cyclic PSI electron transport rates in WT and pgr5 knock-out mutants and revealed that while PGR5 was not required for CET, it was needed for downregulating ATP synthase under high irradiance in both species. Furthermore, in Arabidopsis disturbance of thiol redox regulation by addition of N-ethylmaleimide resulted in downregulation of ATP synthase conductivity in WT but not in pgr5 mutants, and PGR5 interacted with CF₁γ in planta. We suggest that PGR5 functions as a conserved thiol redox state dependent inhibitor of chloroplast ATP synthase under high light, contributing to pmf retention and photoprotection.