A novel extremely thermophilic bacterium (Ni80^T) was isolated from food sludge compost in Japan. Cells were Gram-positive, spore-forming, strictly-aerobic and rod-shaped with peritrichous flagella. The novel isolate grew at pH 6.5-10.5 but not at pH 11.0 (optimum pH 8.0) and 52-79°C but not 80°C (optimum 70°C). The strain could not grow in more than 1% NaCl. The G+C content of the genomic DNA is 72 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain Ni80^T belonged to the genus Thermaerobacter, and that the strain was most closely related to Thermaerobacter subterraneus ATCC BAA-137^T (98.2%), Thermaerobacter nagasakiensis JCM 11223^T (98.2%), Thermaerobacter marianensis JCM 10246^T (98.0%) and Thermaerobacter litoralis JCM 13210^T (97.7%). DNA-DNA hybridization showed relatedness values of less than 10% with these species. On the basis of phenotypic and molecular data, strain Ni80^T represent a novel species of the genus Thermaerobacter, for which the name Thermaerobacter composti sp. nov. is proposed. The type strain is Ni80^T (=JCM 15650^T =ATCC BAA-1731^T).