The bacterium Ruminiclostridium cellulosi DG5, a thermophilic, anaerobic member of the family Ruminococcaceae, was isolated from an industrial-scale biogas plant in Germany. It is able to grow efficiently on cellulose and cellodextrins. The whole genome sequence of R. cellulosi DG5 was established and now provides the genetic basis for biotechnological exploitation of genome features involved in thermophilic degradation of lignocellulosic biomass.