Necrotizing fasciitis is a life- and limb-threatening soft tissue infection. Due to its underlying predisposition and rapid progression, treatment should be started quickly using antibiotherapy and surgical intervention. Although necrotizing fasciitis is mainly caused by streptococci and staphylococci, it may also be polymicrobial. Other peptostreptococci have been reported as necrotizing fasciitis agents in the literature, though we encountered no cases of necrotizing fasciitis caused by Ruminococcus productus. Here, we describe a case of necrotizing fasciitis caused by R. productus, a Gram-positive, obligatory anaerobe.