A photoautotrophic cyanobacterium, Rubidibacter lacunae was reported in 2008 for the first time. The type strain, KORDI 51-2(T), was isolated from seawater of Chuuk lagoon located in a tropical area. Although it belonged to a clade exclusively comprised of extremely halotolerant strains by phylogenetic analyses, R. lacunae is known to be incapable of growth at high salt concentration over 10%. Here we report the main features of the genome of R. lacunae strain KORDI 51-2(T). The genome of R. lacunae contains a gene cluster for phosphonate utilization encoding three transporters, one regulator and eight C-P lyase subunits.