Fusobacterium is a genus of anaerobic, Gram-negative, non-sporeforming bacteria belonging to Gracilicutes. Individual cells are slender, rod-shaped bacilli with pointed ends. Strains of Fusobacterium cause several human diseases, including periodontal diseases, Lemierre's syndrome, and topical skin ulcers.

Although older sources state that Fusobacterium is part of the normal flora of the human oropharynx, the current consensus is that Fusobacterium should always be treated as a pathogen. F. prausnitzii, a gut commensal associated with healthy patients, was completely reclassified as Faecalibacterium (Clostridiales:Ruminococcaceae) in 2002.