SummaryPeroxiredoxin Q (Prx Q) is one out of 10 peroxiredoxins encoded in the genome of Arabidopsis thaliana, and one out of four that are targeted to plastids. Peroxiredoxin Q functions as a monomeric protein and represents about 0.3% of chloroplast proteins. It attaches to the thylakoid membrane and is detected in preparations enriched in photosystem II complexes. Peroxiredoxin Q decomposes peroxides using thioredoxin as an electron donor with a substrate preference of H2O2 > cumene hydroperoxide ≫ butyl hydroperoxide ≫ linoleoyl hydroperoxide and insignificant affinity towards complex phospholipid hydroperoxide. Plants with decreased levels of Prx Q did not have an apparently different phenotype from wildtype at the plant level. However, similar to antisense 2‐cysteine (2‐Cys) Prx plants [Baier, M. et al. (2000)Plant Physiol., 124, 823–832], Prx Q‐deficient plants had a decreased sensitivity to oxidants in a leaf slice test as indicated by chlorophyll a fluorescence measurements. Increased fluorescence ratios of photosystem II to I at 77 K and modified transcript levels of plastid‐ and nuclear‐encoded proteins show that regulatory mechanisms are at work to compensate for the lack of Prx Q. Apparently Prx Q attaches to photosystem II and has a specific function distinct from 2‐Cys peroxiredoxin in protecting photosynthesis. Its absence causes metabolic changes that are sensed and trigger appropriate compensatory responses.