A Gram-type-negative, obligately anaerobic, selenate-respiring bacterium, strain S4T, was isolated from activated sludge of a wastewater treatment plant in New Jersey after enrichment with 10 mM selenate as the sole electron acceptor. In addition to its selenate-respiring capability, strain S4T also respired arsenate with acetate as carbon source and electron donor. Fermentative growth was not observed. The optimum growth temperature was 37 °C and optimum pH was pH 7. Phylogenetic analysis of the 16S rRNA gene sequence revealed that strain S4T is a novel member of the family Deferribacteraceae , with the type strain of Denitrovibrio acetiphilus as its closest cultivated relative, with 91.5 % sequence similarity. The cellular fatty acid profile was composed predominantly of straight-chain fatty acids C14 : 0, C15 : 0, C16 : 0, C17 : 0 and C18 : 0, which distinguishes this organism from its closest relatives. The DNA G+C content was 47.7 mol%. Together, these findings support the conclusion that strain S4T represents a novel genus and species, for which the name Seleniivibrio woodruffii gen. nov., sp. nov. is proposed. The type strain of Seleniivibrio woodruffii is S4T ( = DSM 24984T = ATCC BAA-2290T).