Role of the non-haem, manganese catalase (Mn-catalase) in oxidative stress tolerance is unknown in cyanobacteria. The ORF alr0998 from the Anabaena PCC7120, which encodes a putative Mn-catalase, was constitutively overexpressed in Anabaena PCC7120 to generate a recombinant strain, AnKat+. The Alr0998 protein could be immunodetected in AnKat+ cells and zymographic analysis showed a distinct thermostable catalase activity in the cytosol of AnKat+ cells but not in the wild-type Anabaena PCC7120. The observed catalase activity was insensitive to inhibition by azide indicating that Alr0998 protein is indeed a Mn-catalase. In response to oxidative stress, the AnKat+ showed reduced levels of intracellular ROS which was also corroborated by decreased production of an oxidative stress-inducible 2-Cys-Prx protein. Treatment of wild-type Anabaena PCC7120 with H2O2 caused (i) RNA degradation in vivo, (ii) severe reduction of photosynthetic pigments and CO2 fixation, (iii) fragmentation and lysis of filaments and (iv) loss of viability. In contrast, the AnKat+ strain was protected from all the aforesaid deleterious effect under oxidative stress. This is the first report on protection of an organism from oxidative stress by overexpression of a Mn-catalase.