The morphologies, halotolerances, temperature requirements, pigment compositions and 16S rRNA gene sequences of five culture collection strains and six novel isolates of cyanobacteria with helical, tightly coiled trichomes were investigated. All strains were very similar morphologically and could be assigned to the genus Spirulina (or section Euspirulina sensu Geitler), according to traditional classification. However, the isolates showed significantly different requirements for salinity and temperature, which were in accordance with their respective environmental origins. The genetic divergence among the strains investigated was large. The results indicate the drastic underestimation of the physiological and phylogenetic diversity of these cyanobacteria by the current morphology-based classification and the clear need for new taxa. Three of the isolates originated from hypersaline waters and were similar with respect to their high halotolerance, broad euryhalinity and elevated temperature tolerance. By phylogenetic analyses, they were placed in a tight monophyletic cluster apart from all other cyanobacteria. Thus it is proposed to reclassify highly halotolerant cyanobacteria with tightly coiled trichomes in Halospirulina gen. nov., with the type species Halospirulina tapeticola sp. nov.