The genus Arthrospira is a nonheterocystous filamentous cyanobacterium inhabiting diverse environments including those of high salinity. In the present study, Arthrospira strains were isolated from freshwater and brackish lakes in Myanmar, and their osmoprotective adaptation was investigated as it was for the genus Synechococcus strains. The Arthrospira strains showed satisfactory growth up to 1.0 M sodium chloride, suggesting their acclimation to high salinity stress. Cloning and phylogenetic analysis of ggpS, which encodes an osmolyte glucosylglycerol-phosphate synthase (GgpS), showed that the cyanobacterial strains possess a GgpS-based osmoprotective mechanism, except for Synechococcus strains of freshwater origin. The Arthrospira spp. strains fell into the same cluster in the GgpS phylogeny, suggesting their close taxonomic relationship. One exception was Arthrospira sp. TT-1 (II); the ggpS (II), possibly a paralogue of the ggpS (I), branched out from the cyanobacterial cluster. These findings suggest the wide distribution of the genus Arthrospira in freshwater to brackish environments is ascribed to its glucosylglycerol production as an osmoprotectant and resulting in salt acclimation.