A common bottlenose dolphin (Tursiops truncatus) was presented for necropsy after acute onset of gastrointestinal signs and cutaneous lesions that rapidly progressed to death. Gross and microscopic findings were characterized by locally extensive severe necrohemorrhagic fasciitis and cellulitis, and severe necrotizing myositis in the head and dorsocranial thorax, with numerous disseminated gram-positive cocci. Streptococcus agalactiae was isolated from the lesions and from visceral organs (liver and lung), and it was identified by standard microbiology techniques. This communication is the first report of necrotizing fasciitis in a marine mammal associated with S. agalactiae.