A novel, thermophilic filamentous anoxygenic phototrophic bacterium, strain isl-2^T, was isolated from the Strokkur Geyser, Iceland. Strain isl-2^T formed unbranched multicellular filaments with gliding motility. The cells formed no spores and stained Gram-negative. The existence of pili was described in a species of the genus Chloroflexus for the first time, to our knowledge. Optimal growth occurred at a pH range of 7.5–7.7 and at a temperature of 55 °C. Strain isl-2^T grew photoheterotrophically under anaerobic conditions in the light and chemoheterotrophically under aerobic conditions in the dark. The major cellular fatty acids were C_18 : 1ω9, C_16 : 0, C_18 : 0 and C_18 : 0-OH. The major quinone was menaquinone-10. The photosynthetic pigments were bacteriochlorophylls c and a as well as β- and γ-carotenes. The results of phylogenetic analysis of the 16S rRNA gene sequences placed strain isl-2^T into the genus Chloroflexus of the phylum Chloroflexi with Chloroflexus aggregans DSM 9485^T as the closest relative (97.0 % identity). The whole-genome sequence of isl-2^T was determined. Average nucleotide identity values obtained for isl-2^T in comparison to available genomic sequences of other strains of members of the genus Chloroflexus were 81.4 % or less and digital DNA–DNA hybridisation values 22.8 % or less. The results of additional phylogenetic analysis of the PufLM and BchG amino acid sequences supported the separate position of the isl-2^T phylotype from the phylotypes of other members of the genus Chloroflexus. On the basis of physiological and phylogenetic data as well as genomic data, it was suggested that isl-2^T represents a novel species within the genus Chloroflexus, with the proposed name Chloroflexus islandicus sp. nov. The type strain of the species is isl-2^T (=VKM B-2978^T,=DSM 29225^T,=JCM 30533^T).