Photosystem II (PSII) is highly susceptible to photodamage under high-light stress, necessitating an efficient repair cycle involving degradation of the damaged D1 protein, primarily mediated by FtsH proteases. Although the role of FtsH in D1 turnover is well established, the regulatory mechanisms that ensure precisecontrolled degradation remain unclear. Here, we characterize TEF6, a conserved thylakoid membrane protein containing two transmembrane domains in Chlamydomonas reinhardtii. The tef6 mutant exhibits severe growth inhibition, reduced PSII activity, impaired accumulation of PSII supercomplexes,disorganized thylakoid membranes specifically under high-light conditions. Physiological, cellular, biochemical,genetic analyses demonstrate that loss of TEF6 compromises PSII stabilityrepair. Multiple approaches—including co-immunoprecipitation coupled with mass spectrometry, yeast two-hybrid assays,bimolecular fluorescence complementation (BiFC)—further reveal that TEF6 directly interacts with both the D1 proteinthe FtsH proteases (FtsH1/FtsH2). Loss of TEF6 leads to misregulatedexcessive accumulation of FtsH under high light, correlating with accelerateduncontrolled degradation of D1ultimately disrupting PSII repairhomeostasis. Collectively, our findings identify TEF6 as a crucial scaffold-like factor in the PSII repair machinery. TEF6 stabilizes the proper accumulation of the FtsH protease complex in the thylakoid membrane, thereby ensuring accurateregulated turnover of photodamaged D1. This study reveals a novel regulatory mechanism mediated by a GreenCut protein that maintains PSII quality controlphotosynthetic efficiency under light stress.