Nitric oxide (NO) acts as an important signaling molecule in green algae and higher plants, regulating many physiological processes. However, there is virtually no data on the ability of prokaryotes with oxygenic photosynthesis (cyanobacteria) to generate NO. Although the freshwater cyanobacterium Synechococcus elongatus PCC7942 is one of the most studied model strains, whether NO synthesis occurs in it has not yet been elucidated. Using spectrofluorometric assays and confocal microscopy with NO-sensitive fluorescence dye, we demonstrate NO synthesis by PCC7942 cells. Moreover, we found that the generated NO causes the S-nitrosylation of protein cysteine thiol groups in this cyanobacterium. Notably, S. elongatus PCC7942 utilizes the S-nitrosylation under nitrogen-replete conditions but not under nitrogen limitation. Together, our data argue for S-nitrosylation as an evolutionarily conserved posttranslational modification in organisms with oxygenic photosynthesis.