In bacteria, mechanosensitive channels mediate extracellular release of osmolytes, including glutamate, functioning as safety valves upon osmotic downshift. In cyanobacteria, the role of mechanosensitive channels has not been completely elucidated. Recently, the glycogen-deficient ΔglgC mutant of Synechococcus elongatus PCC 7942 was found to release glutamate extracellularly, giving rise to a hypothesis that the role of mechanosensitive channels in cyanobacteria is conserved. Using the ΔglgC mutant as the model, the present study aimed to examine whether the putative mechanosensitive channel protein MscM mediates the extracellular release of glutamate. Compared to the ΔglgC mutant, the ΔglgC ΔmscM mutant was found to release less glutamate and aspartate extracellularly. In addition, intracellular levels of these amino acids were significantly higher in the ΔglgC ΔmscM mutant than in the ΔglgC mutant. These results suggested that MscM mediates the extracellular release of glutamate and aspartate in glycogen-deficient cyanobacteria. Furthermore, the ΔglgC ΔmscM mutant exhibited more elongated cell shapes compared to the wild type and ΔglgC single mutant, suggesting that the deletion of the mscM gene intensified turgor pressure and/or that MscM is involved in cell division. Through metabolic analysis, the present study revealed that mechanosensitive channel MscM in cyanobacteria is involved in the extracellular release of amino acids.