A Gram-stain-negative, rod-shaped bacterium (strain K-39(T)) was isolated from the thermophilic phase of the composting process for oyster mushroom substrate preparation. The strain grew at 40-80 °C (optimum, 65-75 °C), at pH 5-9 (optimum, pH 7), in media containing up to 1.5% (w/v) NaCl. Phylogenetic analysis based on 16S rRNA gene sequences showed that strain K-39(T) formed a distinct lineage within the genus Thermus. Its closest cultivated relative was Thermus islandicus PRI 3838(T) (96.8% similarity). The DNA G+C content of strain K-39(T) was 71.3 mol%. The new strain could be differentiated from the related taxa by not being able to hydrolyse starch. The predominant fatty acids of strain K-39(T) were iso-C(17:0) and anteiso-C(17:0). Strain K-39(T) contained a lower amount of the fatty acid iso-C(15:0) as compared to related species of the genus Thermus. The predominant respiratory quinone of the new isolate was menaquinone MK-8. On the basis of a taxonomic study using a polyphasic approach, strain K-39(T) is considered to represent a novel species of the genus Thermus, for which the name Thermus composti sp. nov. is proposed. The type strain is K-39(T) (=DSM 21686(T)=NCAIM B 02340(T)).