An obligately anaerobic, spore-forming, acidophilic sulfate-reducing bacterium, strain SJ4(T), was isolated from an acid mining effluent decantation pond sediment sample (pH around 3.0). Cells were Gram negative, non-motile, curved rods occurring singly. Strain SJ4(T) grew at pH 3.6-5.5 with an optimum at pH 5.2. Strain SJ4(T) utilized H(2), lactate, pyruvate, glycerol, glucose, and fructose as electron donors. Lactate and glucose were weakly used. Sulfate was used as electron acceptors, but not sulfite, elemental sulfur, arsenate (V), and fumarate. The G + C content of genomic DNA was 42.3 mol% (HPLC). 16S rRNA gene sequence analysis indicated that strain SJ4(T) belonged to the genus Desulfosporosinus within the family Peptococcaceae in the phylum Firmicutes. The level of 16S rRNA gene sequence similarity with other Desulfosporosinus species was 94.7-96.2%, D. orientis DSM 765(T) (similarity of 96.2%) and D. auripigmenti DSM 13351(T) (similarity of 95%) being its closest relatives. DNA-DNA relatedness values with D. orientis and D. auripigmenti were 16.5 and 31.8%, respectively. On the basis of phenotypic, phylogenetic, and genetic characteristics, strain SJ4(T) represents a novel species within the genus Desulfosporosinus, for which the name Desulfosporosinus acidiphilus sp. nov. is proposed. The type strain is SJ4(T) (=DSM 22704(T) = JCM 16185(T)).