Glutathione reductase plays a crucial role in the elimination of H₂O₂ molecules via the ascorbate–glutathione cycle. In this study, we used transgenic Arabidopsis plants with decreased glutathione reductase 2 (GR2) levels to investigate whether this GR2 activity protects the photosynthetic machinery under excess light. The transgenic plants were highly sensitive to excess light and accumulated high levels of H₂O₂. Photosystem II (PSII) activity was significantly decreased in transgenic plants. Flash-induced fluorescence relaxation and thermoluminescence measurements demonstrated inhibition of electron transfer between Q_A and Q_B and decreased redox potential of Q_B in transgenic plants. Immunoblot and blue native gel analysis showed that the levels of PSII proteins and PSII complexes were decreased in transgenic plants. Analyses of the repair of photodamaged PSII and in vivo pulse labeling of thylakoid proteins showed that the repair of photodamaged PSII is inhibited due to the inhibition of the synthesis of the D1 protein de novo in transgenic plants. Taken together, our results suggest that under excess light conditions, GR2 plays an important role in maintaining both the function of the acceptor side of PSII and the repair of photodamaged PSII by preventing the accumulation of H₂O₂. In addition, our results provide details of the role of H₂O₂ in vivo accumulation in photoinhibition in plants.