Small heat-shock proteins are molecular chaperones that bind and prevent aggregation of nonnative proteins. They also associate with membranes. In this study, we show that the small heat-shock protein HspA plays a protective role under oxidative stress in the cyanobacterium Synechococcus elongatus strain ECT16-1, which constitutively expresses HspA. Compared with the reference strain ECT, ECT16-1 showed much better growth and viability in the presence of hydrogen peroxide. Under the peroxide stress, pigments in thylakoid membrane, chlorophyll, carotenoids, and phycocyanins, were continuously reduced in ECT, but in ECT16-1 they decreased only during the first 24 h of stress; thereafter no further reduction was observed. For comparison, we analyzed a wild type and an hspA deletion strain from Synechocystis sp. PCC 6803 and found that lack of hspA significantly affected the viability of the cell and the pigment content in the presence of methyl viologen, suggesting that HspA stabilizes membrane proteins such as the photosystems and phycobilisomes from oxidative damage. In vitro pull down assays showed a direct interaction of HspA with components of phycobilisomes. These results show that HspA and small heat-shock proteins in general play an important role in the acclimation to oxidative stress in cyanobacteria.